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.
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.