bitcoinj separates the concept of committing a tx to the wallet from broadcasting it. However by default transactions that weren't seen in the chain yet will be announced when a new peer is connected to. It'd take extra code to suppress that, and it's unclear to me why that's useful. I agree with Pieter that it should be the merchants responsibility to get the tx out there, but having the client do the broadcast as well can't really hurt (except perhaps some privacy impact).