Because its one of the ways Invisionfree hosted forums are going about to prevent so much spam on their forum. My friend owns an IF hosted forum and I'm an admin there.. since we made it have to be user validated emails we cut down on half of the spam. Then we just ban emails (I know they can just make a new one.. but most spam sites use something like
ghjh@spamsite.com etc.. so you just wildcard block the email with *@spamsite.com and it seems to stop alot of them)