The trouble is, you are describing different points on a scale with no clear boundary. As anyone into BDSM can tell you, an open handed slap on a bare buttocks is quite capable of inflicting severe pain - and therefore, psychological trauma. Imagine being made to wait for the spank, in a vulnerable and humiliating position. Imagine if it were accompanied by mind games. In short, imagine what a sadist hell bent on inflicting trauma on a child could do if you permitted them to strike their victim, provided they left no permanent marks.

It is true that a malicious parent is capable of mentally damaging their child without recourse to violence, but it does not follow from that that we should shrug and permit it. Violence is an overwhelming force multiplier in an abuser's toolkit, and also strictly unnecessary for good parenting.