How did the boys get into the Nightly Double? Why did they choose to do that?

From the text:

We crossed Sutton and cut around behind Spencer's Special, the discount house, and chased two junior-high kids across a field for a few minutes; by then it was dark enough to sneak in over the back fence of the Nightly Double drive-in movie.

We all had the money to get in--- it only costs a quarter if you're not in a car--- but Dally hated to do things the legal way. He liked to show that he didn't care whether there was a law or not. He went around trying to break laws.