Why, according to Ponyboy, do people usually think that any trouble is the fault of the Greasers and not the Socs?

Here's something that Ponyboy tells us in Chapter 9:

"They [the Socs] looked like they were all cut from the same piece of cloth: clean shaven with semi-Beatle haircuts, wearing striped or checkered shirts with light red or tan-colored jackets or madras ski jackets. They could just as easily have been going to the movies as to a rumble. That's why people don't ever think to blame the Socs and are always ready to jump on us. We look hoody and they look decent. It could be just the other way around--- half of the hoods I know are pretty decent guys underneath all that grease, and from what I've heard, a lot of Socs are just cold-blooded mean--- but people usually go by looks."

What Ponyboy means is that the Socs wear nice clothes and they have normal, socially acceptable haircuts, so they look like they're boys who behave well. But then the Greasers dress differently and wear their hair differently, and then just because of how they look, other people assume that the Greasers are up to no good.

In other words, the Socs don't dress like punks, so people give them the benefit of the doubt; the Greasers do dress like punks, so people treat them as if they really are punks and blame them first for any trouble.

As you can see, the answer to this question leads you to one of the themes of the book: how unfair it is to judge others by how they appear.