Why are grown men struggling to subdue 17 year olds?
People come in different sizes. A grown man can be short, tall, fat, thin, muscular etc. A 17 year old can be short, tall, fat, thin, muscular, etc. The young man in the picture doesn't look too large, but some people are deceptively strong. I haven't seen the cop yet.
It is not a struggle to defeat someone weaker and less skilled than yourself in a fist fight or wrestling contest. It is difficulty to get a person who refuses to do an action to do it. If a person doesn't want to put their arms behind their back, it can be difficult to force their arms to move behind their back. If a person is in a vehicle and doesn't want to leave it, I imagine they could put up a lot of resistance by grabbing hold of something.
An interesting sidenote is that that the injured 17 year old is actually the son of a Kansas City Police Officer.