Based on the fact that the merc's seemed to have only M4's, they probably had no choice but to let them get in pretty close. Between the noise of an outboard motor and the ship, they may not have even heard the shots. The security guys just hop from ship to ship, so dragging heavy weapons along is not feasible. I guess the need to engage them up close is worth the risk as they pirates are lightly armed at best anyway. Thankfully they did not have an RPG with them.