They may have a good reason to be there. That kid had no business on that pitch, he could only be up to no good, so some sort of apprehension was necessary.
That sounds like a very, very dangerous assumption.
I know this might sound kind of funny, but not everyone who breaks the law, for example, trespassing, has bad intentions or means to do harm.
Trespassing, being where you are not supposed to be, is, in my mind, very different from trespassing with the intent to do harm.
Trespassing requires a simple apprehension of the person.
Trespassing with the intent to do harm requires not only the apprehension, but incapacitating the person who is meaning to do harm. But again, there has to be grounds upon which people make the decision that the person intended to do harm (for example, they were waving a gun around and screaming, "I hate you, die!!!!") And they should be able to give their justification.
Tazering someone is a great way to prevent the from carrying out their intent to do harm without actually killing them.
Tazering someone is a convenient, but not very justifiable way, to get someone whose only real crime is being where they aren't supposed to be.
Dealing with someone with no ill intent the same way we treat someone with ill intent, namely, treating a 17 year old in the same way we treat a potential mass murderer or terrorist, sounds very, very dangerous.