Because it could generate more sales of their core products?
I offer free advertising of their products via mine.
What would WotC have to lose?
Only their chance to make the exact same product.
the thing you aren't getting, is what if your product totally and completely sucks?
someone new to the hobby sees it on a shelf, buys it, and it sucks. and they use that as a representative product for dungeons and dragons. and they then assume that all dungeons and dragons products suck as bad as yours does.
that's trademark dilution of the worst kind.
that's why they don't want anything out there with the name dungeons and dragons on it not published by them.
i know the ogl allows free use of certain game elements by the public. this allowed the public to do things like put up their characters and adventures and personal gaming material all over the internet. that's what we fought so hard for in 1994 on usenet.
my concern is that things not covered by the ogl that are in 4e like powers and eladrin, etc., give a right to wotc to prevent you from publishing your own stuff on the web for 4e
i have not read the gsl in any detail, so i don't know what it says in relation to publishing for free your own gaming material. after i post this, i wil read it and see what it says. but still, without a final version, unchangeable as to certain key aspects, i wouldn't trust it.
since i am not going 4e, as opposed to 1994, this time around i dont really give a crap though.