I think companies have it in the contracts they have with artists that they aren't allowed to alter any sketches from the set. This came about after a couple of sets some years back, starting with the 2004 Clone Wars set. Artists (or collectors would commission an artist) would take a poorly drawn light pencil sketch (usually one of the sketches they did for the set, but sometimes a sketch from another artist) and either erase the sketch completely or simply do a heavy full color sketch over top of the old one and resell them. Collectors would buy up the crappy sketches for a few $ and pay $50, $100 or more to commission artists to do full color high quality work. It was actually a thriving business and it lasted for one or 2 more sets after this one before companies cracked down and changed the contracts to stop it from happening in subsequent sets.
Certain artists loved it, they got paid something like $1.50 per sketch from the company but were making a mint altering those sketches. The practice was common enough that these sketches had their own name, they are referred to as After Market Sketches. The more honest sellers of these sketches will refer to them as such, others try to pass them off as pack pulled.
For example, all of Pop Mhan's pack inserted sketches looked something like this:
But the After Markets look like this:
