Because they're not directly making money off of me.
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.
I also refuse to give reviews to Amazon for similar reasons.
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.
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.
> I also refuse to give reviews to Amazon for similar reasons.
I could not picture myself bothered by Amazon reviews. They help decide when to and when to not buy. They also help me decide between two books on the same subject, gives me tips on another titles/authors that I didn't know, etc.
