>>28569576People who think this warning is silly don't seem to understand the danger of an experienced Pokemon trainer in universe
If the SWAT teams set up is any indication, they only have Machamps, Arcanines and Magnemites, any other Pokemon are unknown, and each officer probably has one to two Pokemon on him, and it's unlikely they are anywhere truly high leveled.
Now compare that to a trainer like Giovanni, who would have a full team of highly trained Pokemon, and outside of a 1v1 gym battle, he would use all of them at once, and from experience, most likely be capable of coordinating all six at the same time. So just imagine the kind of mayhem someone like that would unleash, even against a full SWAT team.