Planets with ruins cannot be colonized until the ruins have been explored and looted for all of its (easily obtained and not trapped beneath thousands upon thousands of collapsed office cubicles) shiny trinkets you may or many not actually be capable of hauling off world.
Now obviously I am not suggesting that safety be removed, that would be exceedingly silly. But more for enhanced readability the option should remain visible instead of hidden with a note that the action cannot be performed unless the planet has looted. While most players are going to loot a planet first and repopulate it at a later date it would help newer players who don't realize that they do in fact need to take the loot before claiming the planet as their own.