Cushman was a personal friend of Gene Roddenberry and has the endorsement of his son. What more does he need?
This explains alot. Roddenberry never had any issue with withholding credit and payment for the work of others. Seems Cushman is simply following his lead.
It's not about whether it's illegal or not. Just because an action may be purely legal, doesn't make it right. If this guy is using images that were refurbished by TrekHistory and others, then he should give them credit for their work.
It's as simple as that in my book.