You've sort of identified the problem. Services/taxes/laws/etc. differ from one province/state to the other. So it actually matters where you're a resident (and where you're physically employed--if you are at a fixed office). I guess the alternative would be to have some sort of country-level no fixed residence/federal district status but I can't imagine a lot of demand for it (at least among those who have to political capital to push for such a thing) and I can imagine a lot of abuse if the terms were favorable. (ADDED: And people would just continue to fake things if they were't.)