I don't know about you, but I find that usually if I ask politely to have an item (if I need it for my off-spec) if the other person doesn't need it, they will give it to me.
If the tank in question needs the tank item as an upgrade to their mainspec (tank spec), then obviously they DO have priority over a 6.9k ret, regardless of whatever dps the ret is doing. Being BIS doesn't mean other people owe you for your services. If you want that priority on tank gear drops, then you could farm out some EoTs, buy up some tank gear easily, and start tanking dungeons & raids yourself with dps offset and trinkets.
In a raid environment, the loot rules are what the raid leader says they are, as per the Warmane in-game rules. If you don't like how they're handled, then either find a raid that has loot rules acceptable for you, start leading your own raids, or stop pugging and join a guild.