It's part of the agreement you sign with them. The agreements are not public AFAIK, you'll only see them when you contact someone to assign the copyright. Yes i

It's part of the agreement you sign with them. The agreements are not public AFAIK, you'll only see them when you contact someone to assign the copyright. Yes it would be inconvenient if someone did that, but that's additional incentive for the FSF to make sure they do the right thing.