Because, they have proven that they are not free, their price is your annoyance.
We are all surrounded by so much crap that just to be free is not enough, actually its almost a given for a lot of things esp. on the internet.
However, when free is not enough for a product / service that someone desires, something is really really wrong in that offering?
I am often at weather.com finding out what to wear / be prepared for etc. However, they just lost me to wunderground.com with their annoying dancing lady ads of getting a low mortagage from a particular company.
These ads are such a pain to ignore while you are reading, that they do get noticed (albeit to probably get an adverse effect). These ads have annoyed me enough to not even try them out.
Maybe we should all start a list on delicious or something of these annoying websites (or I need to get some browser plug-in for kind of crap).