Pointer addition vs subtraction

$5.7 -

"[..]For addition, either both operands shall have arithmetic or enumeration type, or one operand shall be a pointer to a completely defined object type and the other shall have integral or enumeration type.

2 For subtraction, one of the following shall hold: — both operands have arithmetic or enumeration type; or — both operands are pointers to cv-qualified or cv-unqualified versions of the same completely defined object type; or — the left operand is a pointer to a completely defined object type and the right operand has integral or enumeration type.

int main(){
        int buf[10];
        int *p1 = &buf[0];
        int *p2 = 0;

        p1 + p2;       // Error

        p1 - p2;       // OK
}

So, my question is why 'pointer addition' is not supported in C++ but 'pointer subtraction' is?