Technical speaking yes, it is. You are only supposed to land punches with the front part of the glove, not the palm where the laces are.
The rules are to use a closed fist You are not going to land a harder strike with your palm . So slapping someone in the ring is not an advantage that will obliterate the opponent its just not part of the proper rules, like clinching. some people try to counter so fast that they dont turn the punch over and land with the front part of the glove. It ends up being an open handed slap instead. Some people look like its being done deliberately. I have never seen someone being told they must punch correctly in a boxing match.
In Europe, punching with an open fist is a no-no in the amateurs, and occasionally gets ruled the same in the pros. But it's rare.
actually if you do it correctly a palm smash is just as hard as a punch if not more so according to Bruce Lee
Wilder hurts lots of his opponents by hitting the side of the head with his palm and sometimes forearm.
