Is using a tax haven legally, or just morally, wrong?

In all the furore surrounding the so-called Panama Papers, it is sometimes difficult to know exactly who are the real crooks and where the real blame lies.
The Panama Papers refer to a huge leak of data (and when I say "huge", we are talking about 11.5 million documents, or about 1,500 times as large as the famous Wikileaks leak of 2010 in terms of actual data) from a secretive Panamanian law firm called Mossack Fonseca. This firm is just one of many in Panama and elsewhere that specialize in "international tax planning", including setting up shell companies (empty shells or fronts, whose sole purpose is to manage the money in it, while effectively hiding who owns the money), bearer bonds (anonymous bonds, useful for transferring large amounts of money around without indicating ownership), etc, in order to help their clients to evade tax, dodge sanctions and launder dirty money. The Panama Papers have already implicated at least 12 current or former heads of state and government, including several dictators accused of looting their own countries, as well as more than 60 relatives and associates of heads of state and other politicians. The leak is still being explored, and more discoveries will no doubt come to light over time.
The Panama Papers, though, barely scratch the surface of the overall problem, and just offer us a small glimpse into the shady dealings going on all the time throughout the world. By some estimates, the total sum hidden away in low-tax, low-regulation jurisdictions around the world could be as much as $21 trillion (about as much as the total annual economic output of the United States and Japan combined).
Many small countries - including Panama, British Virgin Islands, Bermuda, Cayman Islands, Bahamas, Macao, Seychelles, Jersey, Luxembourg, Ireland, and many others - operate as tax havens (or "Offshore Financial Centres"), but people often forget that even some US states (Delaware, Nevada, Wyoming) can be included on this list. While most of the financial services in these jurisdictions are actually perfectly legal, they tend to have a great deal of banking secrecy, weak or restricted regulation systems, and very low or non-existent taxes on financial transactions, all of which makes them very attractive to tax evaders and crooks the world over.
What I have been trying to get my head around, though, is: To what extent is a company like Mossack Fonseca operating illegally? Are tax haven countries like Panama or Luxembourg actually doing anything wrong? Or, does the legal responsibility lie purely with the individuals themselves? What constitutes tax evasion (illegal) and what tax avoidance (technically legal, but often on dubious moral ground)?
All those Third World dictators, Russian cellists and Icelandic prime ministers implicated by the Panama Papers are no doubt morally very bad people for not declaring all of their worldwide incomes for tax purposes in their countries of residence. It seems that funds, ill-gotten or not, can be legally moved around the world from one jurisdiction to another, until they sit in a country that will not give away personal information to the tax authorities of other countries. It then becomes a personal ethical decision for the investor whether or not to make a full disclosure of their earnings.
In most, but not all, countries, though, it is also a legal duty. The vast majority of countries require their residents to report (and be taxed on) their foreign income, as well as local income, which would therefore include income generated in tax havens. The USA and, strangely, Eritrea, are the only two countries that go even further, and also have a legal requirement to disclose the foreign income of citizens even if they live abroad. So, unless you are from a country that either has no income tax at all (like Bermuda, UAE, Monaco, etc), or one of those countries (like Panama itself, Hong Kong, Singapore, Syria, etc) which only taxes local income, then the legal responsibility is clear: not to disclose the income is tax evasion, not tax avoidance, and it is illegal. (If you are interested, Wikipedia has a good breakdown of which countries fall into each of these categories.)
The Panamanian legal firm Mossack Fonseca maintains that it has been operating for 40 year now without reproach, and that it is actually doing nothing wrong, and certainly nothing illegal. As far as I can see, this is true: they are merely fulfilling a legal service for the their clients in setting up shell companies, etc, and they have no legal responsibility to ask those clients what they intend to do with their companies or their money. You can perhaps argue that they might have a moral responsibility, but then you could point the same finger at any number of businesses in all manner of fields for a variety of different reasons. No-one ever claimed that businesses are moral animals.
Do countries have a responsibility to eliminate, or at least discourage, the evasion of tax by foreign individuals or companies? Legally, probably not. They are essentially offering foreign investors and businesses a politically and economically stable environment in which to invest their money, with little, or often no, tax liability, and enhanced secrecy and disclosure laws. There is nothing intrinsically illegal in providing the structure for tax evasion, or even in hiding the true owners or origins of money. Morally reprehensible? Sure! But then so is so much else in the big, bad world of high finance.
So, it seems that facilitating companies like Mossack Fonseca and tax haven countries like Panama are not actually doing anything wrong from a legal point of view. Those individuals who make use of these services and then do not report the income for taxation purposes, however, ARE breaking the law, except in a handful of cases where their country laws allow it.
Either way, though, it's a dirty old business. If tax havens and tax planning companies are not actively criminal, they are at the very least guilty of aiding and abetting.