American publishers in particular (because the stakes are higher than in other markets, the competition more ferocious as well) take the marketing importance of book covers fairly seriously.

Whelan's style is in demand, so when they hire him normally they expect him to do something Whelanesque, but that's another type of constraint in the end, and they would never let him complete freedom with the illustration, the choice of scene etc. anyway. His professional opinion and creative input yes, but the final say on much of anything no. That's part of the job definition of a professional/commercial illustrator.

In this case he had the additional constraint that he was hired because he was capable of creating an illustration that wouldn't clash with the "branding" of WoT, which is exactly what he delivered.

I know it's not you who brought it up, but "being creative" and coming up with a wholly different interpretation or style for WoT, when it's the last book of the series was exactly what they didn't want to do. For all that for many people (incl. me) the art direction chosen for WOT sucks and discouraged me from buying the first book for years because those tacky retro covers could only mean post Tolkien atrocious Fantasy, Tor did manage to create a strong, hard to miss in stores, brand recognition for the series. They'd have been stupid to throw this away at this point, not to mention a lot of hardcover buyers of series are pretty fussy about their matching covers. They're well aware they'll need in a not too distant future to revamp and modernize the image, which they started doing with the ebook covers that we will begin to see in trade format this year. It just would have been a very bad idea to do this for the last HC, especially since their plans for the revamp is apparently to offer the books in trade and mass market PB once the series is over.