One of the study authors here.
On targeting front-desk employees, we did this both to allow relatively portable cross-country comparisons and for reasons of internal validity (e.g., if we placed wallets on the ground, then can participants "select" into the study which compromises our ability to draw causal inferences).
On your last point about whether it's dishonest to hold onto a wallet. The question is about the treatment effect --- all things equal, is it more dishonest to hold onto a wallet with money vs no money? We polled nationally representative samples in the US, UK, and Poland and in all three countries most people thought so.