The thing is, none of these game companies or people have outright said OSRIC is illegal, they have said they have personal reservations about using OSRIC because of potential legal issues. That is NOT the same thing. If it was the former, it would be libel, the latter is an expression of opinion.
From what I understand, OSRIC is using the 3e SRD to justify use of terms and other elements while also using copyright law to "reverse engineer" the older AD&D rule set. I think combining the two is risky. A subset of 3e D&D was released as an SRD under the OGL. AD&D was never released by that, and by licensing the game in the past to Hackmaster they've shown that there is still value in the product. I believe the author thinks the twin legal principles can create a 1e clone without risk. But I can see WoTC getting upset with it. From what I've seen, the "games can't be copyright defense" works better the more differences there are, such as using "health" instead of "hit points", or different terms for the stats.
I can see a reasonable concern for risk. Even if combined these are affirmative defenses under law, a trial can be a crapshoot, and the threat of litigation alone can hurt. So I can see companies avoiding this for those reasons, among others. Sometimes winning a trial is more than just the skill of the lawyers and the wisdom/logic of the judge or jury--it can come down to luck as well.
I don't think WoTC wants to sue, mostly because they don't consider 1e very valuable--if they did they'd probably start publishing a revised 1e ruleset themselves and take advantage of the market. I do think anybody who tries to use the OSRIC method to reverse engineer a 4th edition version of AD&D WILL get sued. However, they will probably never say they will "never sue", as somebody could try to create a game supplement/module that violates the rules they had to create for the SRD after "Book of Erotic Fantasy" came out. In that case, I could see the legal team using every weapon in their arsenal.
I suggest everyone stop giving the impression OSRIC is anything but legal. your really doing harm to the owners of the IP of OSRIC. Intentional or not, Your rumor spreading and fear mongering by just repeating over and over the opinion of "industry experts" (who happen to be in direct competition with OSRIC pushing their own brand of cola). YOU ARE HURTING SOMEONE ELSES CAUSE WITHOUT LEGAL OR MORAL FOOTING. Personally, if I held the IP for OSRIC I would go after some of these clowns for defamation.
I find this a bit laughable because of the IP statement. Intellectual Property? You mean a game who's purpose is to effectively clone the Intellectual Property of Wizards of the Coast? I mean, that's the whole main point of OSRIC, to release a game as close as the AD&D ruleset as possible. I mean, none of the retro-clones would have much value if they weren't meant to emulate the 1st (or 2nd) Edition Advanced Dungeons & Dragons game.