Out of curiosity - do the pages in the book say that these are commissions done for private sale? Also, did the folk putting the book together get asked if it was okay to have a piece of art that you commissioned and paid for included?
For what it's worth - I don't think any of the squaxx who made the original commission would say no to being in a project meant to bring a few galactic groats to the artists family, but it would be interesting to know.
I doubt the artists of the strips would be allowed to make their own collections of strips by Rebellion featuring their work, so what is different here? I see a credit to Rebellion, so presumably this book was done with the copyright holder of the character.