I don't know the details for this, but can talk in generalities.
Often with some professions, when you are hired for a job, there is a non-competition clause that is in effect for a while after you leave a job (especially if you leave by your own choice, but often no matter whose choice it was.)
So, for instance, I had a job that involved creating computer software for use in programming Programmable Logic Controllers (PLC's). A part of my employment agreement was that for 5 years after my employment with that company, I would not be legally able to create or work on software from a competing company, or create my own software that would be competition... ie, software for programming PLC's - for 5 years. I could write other software - say a game or something. Don't remember what the penalty was (a huge amount of money I imagine...)
I also know that inventors when they sell their rights to their product can not make competing products for a time, or someone that owns a company and sells it can't make another company. Sometimes this is only a restriction in that geographical area.
So - with that knowledge - I assume that part of the agreement for the publishing of his Apologia texts - or part of an agreement when he left them - is that he can not publish any textbooks (or maybe just science textbooks) in the U.S. for a specific period of time. That is just speculation on my part, but with the precidence well established.