You can't expect salaries to move (quickly) with currency fluctuations though. A couple years ago the US and Canadian dollars were at parity, and those $70k and $80k salaries would have still been about the same. So not much of a gap at all, especially considering healthcare costs. Yes, at the current exchange rate, Canadian salaries are quite a bit lower when priced in USD. Most day to day expenses are proportionally lower too. (Although not all. Things like electronics tend to be tied to the US price somewhat, although not 100%. Housing is also inflated, especially in places like Vancouver and Toronto, but that's largely a separate issue. You'll notice that, for example, you can buy cars cheaper in Canada right now. (In fact, if you're in the US shopping for a reasonably high-end car, you can almost certainly save money by importing one from Canada.))

So yeah, a lower dollar is somewhat bad for employees who see some of their costs go up (in CAD) due to the exchange rate, as well as those who are planning to spend their earnings in other countries. It's also bad for companies that need to import goods. Of course it's good for companies that export goods, especially those with costs (like employee salaries!) paid in CAD. If the dollar stays low (in a relative sense) for long enough, these things tend to be pulled back into equilibrium, but they never move as fast as currencies can fluctuate.