I think that was a decision made in the last round of testing, no? Because otherwise, once you captured a ship, if you had to re-grapple your original ship it made capturing way too cumbersome. While your crew is on an enemy ship trying to return most of them home, someone comes along and either blasts you with the prize crew penalty or captures/rakes your original ship. Even if you got them back to your original ship, you had to spend more time ungrappling. It just didn't work.

If there was melee, the ships must've been grappled/fouled to begin with, so a quick transfer back makes sense to me. But if another ship comes along which wasn't just engaged in a boarding action, it again makes sense to me that it would have to grapple.