You're almost making a good go of it as the devil's advocate, but so far it seem that they not much stole them, more that they were sitting in them. The report says "Aaron was yelling at the boys about using their beach chairs that they left by the beach access."
Granted, when he started ragging on them they probably flipped him off...
obGrammarNazi: Pretty poor wording in the report. The pronoun 'their' doesn't have a clear antecedant. I'm pretty sure the chairs belonged to Aaron, but they used a plural pronoun in a way that implies ownership by the boys. Maybe the kids dragged their own beach chairs onto his beach access? The more I read it, the less I'm sure.