If they do something to piss off the Canadian public, the Prime Minister could use that as an opportunity to push through legislation to change or even abolish the Senate. Harper, our current Prime Minister, talked about Senate reform in his election campaign, and he's not the first to do so, but it's never been urgent enough to actually do anything about.

A big portion of it is the demographic of the Senate. As senate seats are typically honourary appointments, rather than partisan decisions (though there are exceptions), there is typically little reason or desire to rise up and exert their power. Senators include former career politicians, but also public personas of various origin - athletes, entertainers, and others of non-political origin.

I would speculate that is almost out of a "gentleman's agreement" of sorts that senators respect and recognize their role and place in Canadian politics. Another is attendance - only 15 senators out of 105 are required to make quorum, and many senators are almost never found in their seats.

I shouldn't make it seem as though the Canadian senate is merely an automatic stamp of approval; there have been past examples of dissent, discussion, debate, reform, and rejection of bills arriving in the Senate. However, it is generally understood among the public that the Senate acts primarily in a nearly ceremonial capacity, rather than one of actual weight and importance.