One Piece follows the adventures of an insanely optimistic, rubbery pirate named Monkey D. Luffy and his crew of misfits and reindeer as they travel through the inhospitable seas, battle mean pirates and generally cause trouble for the man.
The One Piece world is as strange and varied as Luffy's crew. Islands inhabited by giants and dinosaurs can be found alongside islands completely populated by transvestites. Trust me, it's not that weird after you've seen a ship ravaged by gigantic sea monkeys.
Your ship makes us go bananas.
These guys are "the man" that I mentioned earlier. Imagine Stalinist Russia crossed with the Legion of Doom with a dash of Hitler and you would have some idea of what these guys are. A bunch of genocidal, information withholding pricks some of whom have superpowers.
They've done a lot of nasty stuff like destroying entire islands. But nowadays they diversify by just enslaving anyone who doesn't align themselves with them.
The government has also been known to make liberal use of super powered assassins(this is the Legion of Doom part of the lecture) to slaughter pirates and sometimes even the hostages the pirates were holding.
Where there is oppression there is resistance. That was from an episode of Justice League I believe...oh yeah! Pirates! The pirates of One Piece are seen as threats and brigands by the stodgy and uncool World Government. Apparently murdering civilians isn't as fun when they do it. Or when they do a better job of maintaining order than the government.
Up yours, authority.
It's sad that most of the pirates are seen as such bad guys when they're really the coolest motherfuckers around.