Because the adventurers are going to people who don't have the apples for their own adventurers, and are takin' their things. Sometimes they'll be so poor in resources that they'll give permission for their things to be taken just to have a few more apples. But the reason they don't in the first place is that the government hiring the adventurers stole all the apples from the other guys, which is how they're able to pay for this kind of thing.

[why is this about apples]