I think this is the same syntax as the one used for casting -- a "type expression" will only be recognized immediately after an opening paren (or at the start of a statement, in case of a declaration statement). So this is rather consistent.
Consistency would be using parentheses for all cases of sizeof, because people without 10 years of experience can pick it up.
If you think something is inconsistent you should ask why and find that what you thought inconsistent are simply 2 different things. The parentheses have nothing to do with sizeof, it's just how you write types (I'm repeating myself).