Ah right, it was PvE. Overlooked that.
Wrong.
Nevertheless it is public and sure someone can call it theft but someone can call it public. Always a tricky topic. Especially when the owner of the structure is not near the ship.
If the owner is near and someone steal it straight away it would be not cool and we would step in with setting him guilty etc.
Here it was just your own mistake.
Only thing what I can help you with is a little boost in your inventory. Have fun.
In the end it was lesson learned for you for sure to never ever set stuff to public since everyone can access it.