The sandbox is suppose to give players freedom of choice. Those choices whether they're positive or negative consequences should cause the player experience a positive or negative reaction. For example, if a player decides to kill another player, the consequences of his action should be negative. It maybe possible that other players in the area may decide to kill him/her for their treacherous act. Or another possibility is that player may receive permanent damage to his/her reputation from the game's community.
A long story short, a sandbox is where players or the community set the rule and limits on what is possible. The game design does not interfere and say, "not we don't want you playing the game that way." A sandbox is when players are capable of playing in countless different ways, creating their own content and story. Full loot pvp is simply a part of what a sandbox MMO has to offer, but it does not mean that full loot defines the definition of a sandbox MMO. Just because there is full loot pvp it doesn't mean that the MMO is a sandbox.