As I see it, their IM servers are theirs to do what they want with. If they want to block 3rd party IM programs that's perfectly within their rights.

A few months ago Yahoo! changed their authentication some which caused some trouble for Gaim and other 3rd party clients. The Trillian developers eventually figured it out and donated some of their login code to Gaim.

In reality, I don't think any of the IM networks care enough to do anything. I saw an interesting quote in an internetnews.com article, "Tools like GAIM are in clear violation of our terms of service and our copyrights" (http://www.internetnews.com/dev-news/article.php/3363481).