I would have assumed that the artist retains copyright on the work, and is effectively selling you a copy or a print of something they can reproduce if they wish to.
Again, not if it's an image of something they don't own the copyright to. In most instances, a drawing of the Millennium Falcon/Serenity/Enterprise A through Z would have more value to a commissioner than "random spaceship the artist made up in their head" and that value in part comes from the IP that someone else owns.
An artist can draw anything they like, but
at the point of monetising the work then, if it's a pre-existing piece of IP or demonstrably derives from one, then ownership of that IP comes into play and the artist
should only sell it to the owner of that IP, or to a third party with the owner's explicit permission, for which they would be entitled to demand a contract and a cut of the proceeds reflecting the added value.
It's grey area because of the long-established precedent of publishers/licensors turning a blind eye to one-off commissions and
very short run sketchbooks and prints, but different IP holders set the threshold for where their tolerance runs out at different levels.
