I think that's a stretch in this case. The authors have a definite choice. They don't like it then they don't have to write tie-in fiction.
"If you don't like it, you don't have to take the job" has always been the excuse the capitalist class uses to justify exploitation of the working class.
And they get compensated for their works to begin with.
They get compensated for their labor in producing the novel for one company -- but then they don't get compensated when that labor is used to produce an entirely new work by a different company, thereby allowing that company to profit off of their labor with no compensation. That's exploitation.
It's not like the authors are working in a sweatshop for pennies on the dollar, which could then be exploitive.
Your concept of exploitation is too limited.
And I wouldn't even call it contract trickery.
Neither would I. But you seemed to be saying it's only exploitation if the employer does something not specified in the contract ("contract trickery"), and I'm asserting that everyone can know exactly how everything works out in a contract before they sign it yet it can still be exploitation.
Because what makes it exploitative or not is power, not contract trickery. If the employed do not have sufficient bargaining power to obtain better terms and if the terms of the contract are unilaterally dictated by one party, then there's usually exploitation going on.
I think it's overstating things to say the authors of Star Trek novels are being exploited.
I don't think any other word is appropriate for an author's labor being appropriated by a third company to financially benefit them without them compensating that author.
To put it another way: If Company A contracts out the job of writing Software Program 1 to Company B, and Company B contracts that job out to Jim, and Jim writes that piece of software, and then Company A takes a feature from Program 1 and uses it in Program 2, and Program 2 brings in way more money than Program 1, then Jim has been exploited. Sure, Jim was compensated for creating software for Program 1. He was not compensated for his labor being used in Program 2.