True, but in languages with operator overloading you (may) end up with other issues. Unless you look at the source you have no idea what the '==' operator will do.
I have always wondered why all languages don't enforce this?
Because it is often really convenient, particularly with a simple macro preprocessor like cpp but in other circumstances too. I like gcc's approach of forcing you (if you have the right warning enabled) to put an extra pair of unnecessary parentheses around assignments in expressions.
because, for example in C, such a incompatible change would alienate most developers used to the very ideomatic