We agree! If you’ve had a gray item deplete a site, this is not intended and is a bug. Please be sure you file and in-game /bug to report this if it happens, and we welcome follow up posts with details for us to investigate.
This past weekend I visited the three sites on Alderaan and dug up items. While in the House Trader's Circle site, I dug up several items that the system "found interesting" that turned out to be grey items. I don't remember specifically the item names, however, and I've since sold them. I can tell you this - when the items were dug up, they didn't come in the treasure box; they looked like the grey items. Also, I received several green GSI rep tokens as interesting treasures, as well as, green seeker droid boosts. Out of the 4 (or 5) special treasures that depleted the sites, I never received a grey item or a green item - they were all purple items (purple GSI rep token, pleasure speeder part, artifact level 50 roll box, etc.).