I suspect it comes down to the contract Dave Pulver had with ICE. With "For Hire" at one end of the spectrum and "first publication only" at the other, even a supposedly clear copyright statement in the SpaceMaster books is of variable enforceability.
Indeed, it was a contractual issue.
* ICE indicated they would pay the author for Aliens & Artifacts.
* They suffered financial problems in the early 90s and neglected to pay many of their authors and licensees, including myself.
* Their contract stated that rights reverted to the author in the event of non-payment.
* I was one of many creditors who ICE owed money to when they went bankrupt. I never received money, but was sent all the bankruptcy deal paperwork, and as part of the legal settlement of ICE's debts I received a written letter from ICE's representatives returning all rights to the material (with the exclusion of any references to ICE trademarks that appeared elsewhere, e.g., their background).
* As far as I know, no Spacemaster product other than Aliens and Artifacts used the material; if so, they were not supposed to.
* SJ Games were fully aware of every element of this when they hired me to write Alien Races; they acquired all rights to those races that were published. The parts of the book that did not appear in the Alien Races I retain rights
to (e.g., the creatures and the couple of aliens we thought didn't fit into Traveller).
* I think but cannot confirm that Far Future Enterprises was acquiring the SJ Games material following the end of the license; if that is correct (and I don't know whether it is or not), FFE would now own the rights to the aliens, and would be free to do whatever they want with them, from making them go aware to rewriting them to retaining them in a separate Lorenverse sub-canonical setting.
* Don was fully aware of all these details, thanks to correspondence we had on the subject when he asked me about it last year; whether they were of interest to or passed onto Marc I have no idea.