There is an important legal principle in First Amendment law, that is that the argument that "Well, this law COULD apply to a lot of things, but we're only going to use if for really nasty stuff!" is bunk. In short, the assumption that an overbroad law is acceptable because, wink-wink, nudge-nudge, we won't use it everywhere we can, is bogus.
Now, the 1A doesn't apply here, since this is a private contract, but that principle, as applied by those who judge the moral/ethical (as opposed to legal) standing of these changes, certainly does.
"We COULD screw mongoose...but we won't! Honest injun!" is bullcrap. And even if the person saying it meant it, it doesn't mean the next administration would abide by it. If the only justification you have for a grossly overbroad clause is "I'm sure they'll only use it against the bad guys!", then, you have no justification.
Does WOTC have the legal right to make this change? Sure.
Is the change moral, ethical, or supportable by anyone with any sense of decency or fair play?
No.
It's a direct attack on an ex-employee, and it strikes (as Ryan Dancey and others have noted) directly at the heart of one of the main benefits of using the D20 STL. It opens any and all D20 products to arbitrary and unjustified legal assault by WOTC at any point in the future, without any option to mediate or correct the breach. It is ridiculous, and the long term effect will be devastating to the D20 -- but not necessarily the OGL -- community.
WOTC screwed up, big time. If they are smart, they will pull back and rengineer this clause to be far less open ended. If they are NOT smart, expect to see OGL products outpacing STL products pretty quickly. Using the STL has simply become unsafe for *any* published, regardless of "adult" content, simply because of the subjectivity and overbroadness of this clause.