Only because the advertisers will complain, and because it's a reason to get a mac [1]. Say Best Buy removed the crapware ads for free, and the advertisers didn't complain. I'm sure Dell couldn't care less, as it's actually a better experience for the user.

[1] As far as I know, no one offers "optimization" on a mac, so buying one is less annoying. EDIT: this is wrong: http://news.ycombinator.com/item?id=1032781