The customers (advertisers) actively push for insidious installation methods. Legitimate software developers don't advertise using those methods (have you ever seen an ad for WinEdt?), preferring google or other direct methods.
So the population IM attracts are precisely those unscrupulous developers and programs that profit from questionable behavior. Working in a proper manner runs counter to their customers' goals.
Legitimate software producers do use similar methods I outlined above. Examples, and correct me if I'm wrong, my memory is quite shaky on this subject, ubuntu installer, steam installer, most PC games, and I think even the open office installer uses similar.
The examples are usually just advertising other products they own, or features within their software so not exactly direct examples but its not a huge jump by any means.
While I agree the population of customers they will attract are those shady toolbar guys, it certainly doesn't follow that they cannot make a business out of pushing good software. I think as long as they tailor the ads to relating software or software people will likely install alongside the current piece it could work. Taking the high road could even payout bigtime down the line when more developers catch on and realise it won't detract from their image/brand by using such a service.