I'm not even sure why they made this change. Nothing ever targeted unoccupied vehicles before now, and it was fine. They even removed the targeting from the Solo Silo because, surprise, having your defenseless gear get destroyed before you get to play with it isn't fun! Then the Bastion came along and suddenly every enemy in the game has a burning hatred for hunks of metal just sitting around doing nothing.
Seriously, why did they change this? There is nothing in this game more infuriating than getting out of your brand new mech to punch a code into a terminal, getting sniped by a lone hunter that was hiding behind a rock and running back to find your incredibly high cooldown stratagem got slowly picked apart by three scavengers and a warrior while you couldn't do anything to stop them.