There are cops trying to stop the evil "teams", but I guess they just aren't skilled enough to take down all those level 20 Zubats and Grimers. The reason the player is always the one to thwart the teams is because the player is stronger than the region's law enforcement, I guess. Which really doesn't make sense because how would the law enforcment control you or other strong trainers if you went bad?
No, I think law enforcement in Pokemon is only meant to deal with minor problems like petty thiefs. Like when your rival stole the starter Pokemon in G/S/C, HG/SS, a police officer came to investigate. But more major problems are left to gym leaders, elite four, and once and a while a very strong trainer(the player). In G/S/C, HG/SS, Lance, the champion of Johto helps defeat Team Rocket when they come to Johto, respectively.
Well. . . you are the only one. Most of these plots were secret. The headquarters were all secret, and in every instance, they never got around to doing anything really bad because the player barges in and stops them. Police need warrants and reasonable suspicion, and these teams are secret. The player stumbling in on that secret is merely a plot device.
But I have to agree with you more than what I said.
As for the leaders...I don't think there are leaders in Pokemon. There are corperations and services(like law enforcement), but financially, I think it is more simple than the real world. You buy with money, and you get paid for work(or winning Pokemon battles). There is no financial aid, because everyone in the Pokemon world seems nice enough to help their neighbor out. If you needed more money, you could train your Pokemon(which does not require any money) and win a bunch of battles. In this system, there is no need for a government. People can take care of themselves within their own communities. With no government, there are also no taxes. One of the major points of the Pokemon world is that people live simply and without much worry beyond their own little niche in the world.
Sorry for the long post- it's probably because I love sociology so much...I just had so much to say!