Sierra isn't nearly as malicious as LucasArts. In any case, their (LAC) legal department is flat-out wrong here, as would be Sierra's if they challenged FreeSCI/Sarien. If Sierra was ever actually a member of the IDSA, they aren't now (which is something that bugs me to all hell about HotU -- all (err, most, though that might just be because Dynamix is supposedly the copyright holder for that one, despite the fact that they were a subsidiary of Sierra) Sierra games are supposedly protected by the IDSA even though they aren't a (listed) member of the group).
Aaanyway, like I said, I think Sierra is in general a more intelligent (and less greedy) group than LAC. (Gee, hope I don't get sued for deflamation of character or some bullshit like that for saying that.)