This is where all the big stuff starts happening on day 2. If you burn that time in DHS you're really setting your money on fire. Now if someone got stuck out there and needed to get back somehow, I'm sure they'd run a one off shuttle to return them to the "ship".
Are big things happening on the Starcruiser from 8pm until 9:30pm? That is a time they could have people willing to play in Batuu since that park is open until 9pm on tonight, such as tonight, and probably until ten soon in the summer. This will allow more play time for those side missions and rides as well.
the 4pm ending shuttle time is purely for Disney's convenience.