I would be pleased to have other opinions on this.
If Adwords is doing marketing to the Canadian market, they obviously want to have set prices, not "signup is $5 US, which will be converted at the daily exchange rate".
Not only that, imagine the hassle if they had to update pricing across all the different currencies (Canadian isn't the only alternative currency they offer) which have to be updated everytime the exchange rates fluctuate.
If you feel the US pricing is a better option for you, then go ahead and signup with US pricing instead of Canadian. Do keep in mind that you cannot change currencies within your Adwords account (unless this has recently changed), so choose the one you would prefer to use.
<added>It does specify the currency, but most people just see the dollar figure</added>
The exchange rate is much different than what it was when they started accepting Canadian currency, so this probably makes it even more noticeable for Canadians who pay attention to those kinds of things.