Oh, see, I was right!
I remember one of the 4kids Pokemon VAs mentioning this once after they were fired from Pokemon, like they legally couldn't go to a birthday party and do Meowth's voice, even though it was their own voice, because of rules like this.
I worked for Disney in one of the hotels back in college. I wanted to get a little card signed by Princess Aurora as this mom bought an Aurora toy for her daughter's birthday and was having it shipped home. Thought it would be a nice surprise when they opened the mail. I had to find someone that had a certificate and permission from Disney that allowed them to sign as one of the characters. Very protective of the image. Luckily one of my managers did