Because it sends completely the wrong message to everybody involved; it quickly fosters an us-and-them mentality that isn't good for grass roots sport, which is fundamentally about a bunch of people coming together for a few hours to have fun. Yeah, we keep score and have a result at the end, but the most important thing is that people enjoy themselves. Also, my picture of grass roots means that the teams don't have much money washing about and to hire trained security would be a completely disproportionate expense that'd make most of the teams very uneconomical very quickly.
Possibly our definitions of "grass roots" differ; for me, you're no longer at the grass roots once you need stands for the spectators (and so, a lot of HS ball in the USA is past grass roots). That, more than anything else, changes the equation and makes it appropriate to have an overt security/stewarding presence, and to strictly enforce things like mandatory cooldown periods after the game ends.