Giving the US more ability to spy on us is still a bad thing, regardless of what capabilities they already have. That's like saying that you should just give burglars all your stuff because they could break in and steal it if you don't.
And it's a completely valid reason for people to be mad about this. Is it the main reason to be mad about Bill C-22? No, of course not. We all know that. But if focusing on this minor, tangential issue is what gets the average person to give a shit then I'll take it.