We've discussed it before in this forum, but the key is representation, and whether there is an issue with actors of that category getting work, which is worsened by casting people outside that group to play them.
Disabled actors fit that definition - they are normally overlooked even for disabled parts. For example, the prominent disabled character in Glee was not played by an IRL wheelchair user, and for no real reason - the character is always shown in the chair.
Mothers and Americans probably don't. There's a sliding scale.
I find it helpful to think of it by flipping it around. How likely is it that a production will hire an actor who uses a wheelchair for a character that doesn't? So if parts for characters in wheelchairs also go to actors who don't need them, you've pretty much pushed disabled actors out of work.