Because they're not directly making money off of me.<p>I used to comment on movies on IMDB until they took the information I and others submitted to their free website and went commercial, putting tons of ads up and clearly started making money off of my contributions.<p>I also refuse to give reviews to Amazon for similar reasons.<p>I'm ok with participating in non-commercial forums, as I feel they are less likely to sell information about me than sites which have gone commercial.<p>Also, I don't really view forums as tools for collecting information about their participants, unlike all the "free" web-services run by corporations.  I suppose forums could be used for this purpose, but I think at this point it's just less likely.  But it's definitely something to consider.