I can't tell you exactly why but it's consistent with just about everything else involving p being undefined, and the result of the comparison would be useless anyway.
I can imagine situations where a pointer q might sometimes be a copy of pointer p and sometimes might point to something else, and the code wants to free q if and only if it is not a copy of p (because p has been free'd earlier).
Because a new object can have the same address as p, so comparing to p isn't enough to tell you if you have a copy of p or a live pointer to something else.
I can imagine situations where a pointer q might sometimes be a copy of pointer p and sometimes might point to something else, and the code wants to free q if and only if it is not a copy of p (because p has been free'd earlier).