How can an MP hold down more than one job?

Liberal MP Raj Grewal has resigned his position in order to deal with a serious gambling problem.
Initially, the word was that he was resigning for unspecified health reasons, but it turns out that the first-term MP for Brampton East has racked up over a million dollars in debts from gambling, principally in Quebec casinos. And this comes on top of his previous ethics and conflict-of-interest controversies, such as his invitation of a company with which he has a business relatonship to the Prime Minister's trade trip to India back in February of this year. One can perhaps sympathize with someone who suffers from a gambling addiction, although less so with someone who has the bad judgement to get into this type of conflict-of-interest difficulty.
What I find even more egregious, though, is that Grewal has apparently been holding down not just one but two other jobs at the same time as his supposedly full-time gig as a Member of Parliament. Grewal is also employed by Brampton contractor Zgemi Inc. (yes, the same company for which he managed to obtain an invitation to the India trade shindig), and by law firm Gahir and Associates. I know that, if I were a constituent of Brampton East, I would be pretty pissed off that my MP was off moonlighting, and not devoting his full energies to the well-paid job that I helped to get him.
It turns out that, although minsters and parliamentary secretaries are barred from engaging in outside employment, back-bench MPs are under no legal obligation to give up other jobs. They do have to disclose the fact, but not the specific company or line of work, or their remuneration, or indeed their time commitments.
Call me old-fashioned, but it doesn't seem like too much to ask for an MP to devote his full attention to the work of running the country, and representing the constituents who voted for him. Does it? And having more than one boss sounds like a sure-fire way to end up with a conflict of interest. Oh, wait...