I believe he says this because they are, after all, somewhat of criminals.
Many of the kids are both good and bad, but there is not one of the Greasers who is not willing to participate in illegal acts for no really good reason. The Greasers are all willing to go fight with other gangs, to beat up Socs who come in their territory, stuff like that.
So, they tend to go out looking for trouble, stealing things, doing things like that. So even though they have (except for Dally, perhaps) good sides, they also do a lot of stuff that makes them deserve trouble.
In the book "The Outsiders" the Greasers are known as having engaged in inappropriate things. When Pony Boy describes Dally he demonstrates some of the things that happen. Dally steals, causes fights, rolls drunks, and picks on kids. Two-Bit attends school but carries around a switch blade ready for action, and he misbehaves in school. Soda Pop dropped out of school and gets speeding tickets. Steve Randle, Soda Pop's friend steals hubcaps from cars. Even the younger boys drink as Pony Boy pointed out.
"It's rare to find a kid who doesn't drink once in a while."(8)
Pony Boy does not want people to think that the boys are totally innocent, but he will later explain that they are not totally bad either.
well what happens was that pony boy when he is writing his story he says he doesnt want to be a greaser