In my case, I think something must have happened because that used to be the case. You might be able to jump through some hoops to avoid that (never tried), but the funny thing is that I didn't do anything with my "professional" account for it to automatically switch back to SMS. The first time it happened, I was actually surprised and thought something fishy was going on.
I'm not aware of any such law. What I suspect, is it has something to do that the app can only connect to one account, but in my case I have two (personal and pro), so it must have some issue handling both. I freelance in IT, so there's nothing too interesting going on in my professional account, so I expect them not to care too much, since I have the app for my personal account anyway.
