[Baron Munchausen]

This is an intentional feature of planetary shields. It was the same in SE III, btw. This is one of the ways that MM tries to encourage use of troops. Now, if only he'd change the way troops are handled during bombardment so you couldn't destroy them from orbit when there are no shields or the shields are down. If they could be stored in a 'special' way and only damaged gradually like facilities are damaged during population reduction you'd be pretty much forced into heavy troop-to-troop combat if you wanted to capture a planet in relatively good condition.