More accurately, big retailers wouldn't. Like, people are paying more than half the cost of the base figures to add stickers or millimeter-width sheets of plastic to them.
Hasbro-the-company probably (hopefully) doesn't give a thought to third-party add-ons. They care about how much they spend to produce "units" and how much they can sell for.
The people who actually design the toys (many of whom, at this point, are fans and collectors themselves) might well be more aware (and supportive) of third-party groups.