Why is Amos angry at the boys in Act I?

The boys took it upon themselves to interfere in Amos' business after eavesdropping on a conversation.

AMOS: So let me get this straight. You overhear a conversation — a conversation which was not meant for you to overhear — and you decide — without prompting, in fact, without leave — to interfere, and interfere hard, in someone else’s business.