The language of Mathematica is a formal language and however intuitive it may seem
(it does seem so to me) the basic principle of such a language is "everything is forbidden
unless explicitly allowed" rather than the opposite.
The semantics of Mathematica is not formally defined, and its apparent intent is to do as much mathematics as can be automated. The exclusion of certain operations on certain operands may change without notice in the documentation.
If your statement is meant to be read as "unless explicitly allowed [by the proprietary and secret source code]", then it is vacuous. Of course the programs do only what the program do; this is true of any programs whatsoever.
Actually, the syntax of Mathematica is not formally defined either, but at least that doesn't change so often.