Such slap contests were popular in Russia about 3-4 years, specially on YouTube. I know that there are also slap contests in UK and Poland. I personally does not count this as a sport at all. You are right that this is extremely dangerous. Power Slap will close as soon one of the contestants get a brain injury, gets a serious concussion or dies from cerebral hemorrhage.
The problem with Power Slap is that there are no clear rules how strikers can slap. It is only said that they can slap with open hand basically. In Russian version of slaps, it was allowed to slap with fingers only, straight, unbended in elbow arms. And they were allowed to slap only in the cheek or chin. Otherwise you get a disqualification. In Power Slap, they mostly slap with edge of palm (dont know how it is named correctly in English, it is called "abductor digiti minimi" on pictures) to the temple. Or the reffs are suck and warn for such wrong slaps. Anyway, it is like a hook the head. A hook that it is not allowed to block or dodge. This is not sport!
Yeah, I already saw one of this video long time ago and the fat guy is always starring there. I think the guy is Russian. Could be true since you said it was popular in Russia. The hype then died after some time but I am surprised to see a topic like this, only to know that this sport was still active and even some betting sites already have this now.
Maybe not all games have the same rules and that is why someone can get damaged heavily but they better refrain from doing that, and if not then someone can report them to the authorities. After all, people only want to get entertained but seeing someone die or suffer badly isn't fun anymore. The audience can get guilty too.