Well, there's several reasons.
1. I'm sure 85% or better of the members here don't care about stamps so why weigh down the database with all their info?
2. Personally I think it's a tad unethical to sign people up for something they didn't ask for or agree to.
3. What if someone sends them an email through the forum and they didn't sign up? That email is automatically spam, since they never registered for the forum.
4. No one that I transfered will have agreed to the terms, conditions, and legal disclaimers for Stamp Community.