To me, a "spank" is usually a mild to moderate slap on the rear, a part of the body particularly well cushioned that can handle it just fine without physical trauma, not to mention psychological trauma.
When I think of a child who has been "beaten", I imagine repeated bludgeoning across the body, often with a tool, and usually with visible bruising and possible laceration and bleeding.
If you've ever been a parent, regardless of whether you think spanking is OK, you know that there is a huge difference between these two activities. I personally avoid physical discipline myself, but I can understand a parent who decides to give their kid a spank on the rump after the child did something where they didn't understand how seriously negative the implications were.
For example, if 5 year old runs out into traffic on a busy road and you pull them back, which form of discipline is more likely to negatively incentivize the impulse to dash into the street to get a penny? I can see why someone would think that the short – but sharp – pain of a quick spank is more effective at protecting the child's short and long term health.