It seems more likely to me that yes, they could do a DELETE FROM BROWSING_HISTORY WHERE USER = "%S", but that's not their business model. Instead they have to add a column to the database that flags data that will be shown to the user or not, and then go through every part of the site where the data might be visible and add a check for that flag. It's a lot more work, and worse they have to get it right the first time because people will be angry if there is a leak somewhere and their supposedly deleted data is visible.