Follow and Block make sense as commands you can send through a message. But Accept? Why would you ever be able to control an action on someone else's account? It's rather odd that this exists at all.
If you've got your tweets protected (private), you have to accept users to allow them to follow you.
I think you've missed the point here. This isn't a command that tells your account to accept follow request, or adds someone to your following list - this is a command that instantly makes other people 'accept' a follow request from YOU. This works completely differently in how it would consider the username parameter, and in that the change is applied to the other person's account, not yours.