Interplay isn't taking anything they aren't entitled to, if Bethesda wanted the distribution rights to the old Fallout games they would have included them in their offer. If they wanted MMO rights they would have included that as well. As per the contract they signed, Interplay retained the rights to distribute the original Fallout games on PC as well as the rights to do any MMO Fallout thing.
Also, Bethesda receives royalties on all of this stuff anyway so it's not like they are left on the sidelines here while Interplay reaps the benefits.
And the Fallout Trilogy was selling well before Fallout 3 released, it's hard to deny that new people may have discovered them as a result though and I think this is something Interplay banked on with the MMO.
Lastly, given the e-mail messages on the Vault predating the lawsuit, I'm guessing Interplay must be raging Hulk mad at Bethesda.
I really don't understand where these type of theories come from, this entire ordeal is pretty well documented and most of the big gaming sites seem to have covered the issue pretty well yet you will always have people spouting misconceptions on various comment pages, threads, etc (not directed at you btw, just something I've noticed in general about the case).