Colleen at a Simple Kind of Life blog writes that WordPress.com doesn’t want to become home to spam bloggers and/or folks who make money off of advertising (she has an email that says that PayPerPost advertisements aren’t allowed on WordPress.com).
WordPress.com’s Terms of Service are very clear on this point, even highlighting it in blue.
This is a good thing for WordPress to do. Why? It protects its reputation. PayPerPost is a way to game search engines. If you want to do that, take your blog somewhere else and protect those of us who aren’t willing to do that.
Also, if you want to put lots of ads and things on your blog, why are you using a free service? Pay for a host and put that stuff somewhere else.