I would neither like when my son were to participate in such a competition as a slapper nor would I like if he was standing around screaming like a cheerleader when his favorite slapper humiliates his opponent. That is just stupid. I don't like boxing either because I think that our brain is so valuable, it allows us to do so many things, feel so many things and so on. Why get it slapped permanently until it might take damage and stop working accurately or maybe even lead to death? That is just not my thing I guess.
Every professional sport causes serious injuries to athletes. Soccer players often have problems with their knees and feet, jumpers have problems with their spine, boxers have problems with their head.
To avoid serious injuries, you can practice boxing for 2-3 years. After that, you can just keep in shape. Such training makes a person very tough and strong in spirit.
But there isn't most likely even a decent ratio in slapping tournaments in terms of risk and reward. When you trash your knee as a professional soccer player for 20 million EUR in your career, well fine, and you usually don't have any problems with your brain. The goal is to score goals, not to trash someone else's face with a single hit for a couple of bucks, risking the other person's health. But I might just be different though. You know these fail videos where people break their bones in gymnastics and so on? I can't watch them, but I know a good number of people who can laugh about it, I just can't. ;P
Every sports involve some risks of serious injuries to the body, but in fighting sports the risk is much higher and the consequences much worse, because as you said, people are hit on the head, the most important part of the body and probably the part people most worry about protecting against injuries.
I can't say slapping doesn't call my attention, especially in slow motion when we can see all the movements of the player when slapped, but it's really not a good thing for them. These players seem young right now, but in some decades they will face a lot of issues with their mental health. I've seen famous boxing players suffering from these issues and it's really sad. With slapping participants it must not be different.