Although Luffy is a potential, right now he's supposed to be a little weakling in the world of the mighty pirates. Wouldn't make much sense to let him have a bounty twice or triple as high as the other characters.
Why do people keep pushing this "Luffy is weak" stuff. Yeah, next to the most powerful people in the world he is. But we've been told since Enies Lobby how notorious he has become. He's also taken down two shichibukai and Lucci. And while the New World has the strongest pirates, we've seen through Hawkins and Kid that not all of them are. No, he's not near Yonkou level, but I expect them to have insane bounties anyway.
And we've been shown repeatedly that bounties mean very little. Robin had one as a defenseless child, while Blackbeard didn't have one. It'd be silly to not raise Luffy's bounty considerably after he's all he's done directly opposing the government. He's a direct threat to the government, unlike other pirates, and just about every marine has called him that.