Nailed it on the head.
Its the same with modifying cars, tint, exhausts, etc. are all things that can be illegal its just whether or not police are busting down on those things. Gotta pay to play!
But most aftermarket undertail kits that come with a new bracket are basically in the same place as the stock one just a lot less gaudy.
Another thing is people that bend there plate because they directly mount it to the undertail, which I believe it is also illegal to bend a plate.
To the OP, where have you been getting stopped? Same area? Just wondering as I have a good friend who has his plate mounted in the same spot and hasn’t been pulled over in the 3 years he has had it there.