Hmm... it is intentional this is disabled but I can't think of the particular reason off the top of my head. Perhaps it was done for future compatibility where vector shapes might not be able to work correctly as buttons, at least with the current implementation. The workaround you found is an interesting one, but I'd probably consider it a bug as it stands right now.
@graphicsgeezer it sounds like you just need something that only has specific click regions and doesn't need the button states, correct?