I don't believe that most ISPs intentionally block legal competitive products and services. I think this is just another example of the side effects of using "protection software". Just like [link|http://www.wired.com/news/privacy/0,1848,43576,00.html|this]:
AOL had previously contracted with SurfControl to operate its parental controls. While RuleSpace's system will be mostly automated, SurfControl's SurfWatch -- like many filtering products -- relied on human editors to make the final call on which sites should get blocked.
Automated and manual types of analysis both have drawbacks. A team of editors can only review a tiny fraction of the seven million new websites added to the Web every day. But automated filters have become notorious among civil libertarians for "over-blocking" sites based on just one blacklisted keyword. For example, the ACLU likes to point out that conservative Congressman Dick Armey's website is blocked by a popular filter.
The way to avoid these problems is to not use filtering software like this.
FWIW.
Cheers,
Scott.