Does that include drip systems? Of course. What would make you think that drip would not have the potential for backflow contamination that regular does. And if you are going to claim that the emitters are up in the air so there is no way for water to flow back into the emitters, you're forgetting about damage to drip tubing becoming a source of contamination.

What does a T connection above the device have anything to with it?

You are dealing with fluid dynamics. I don't have the time to explain everything I know about fluid dynamics, but lets just put it this way...

Based on the instructions you state the backflow device gives, that means it is the type of backflow that can not guarantee to prevent backflow if there is back pressure (i.e. water being forced backwards into the device). This is why the instructions state that it must be 12' above the sprinkler system... because water can not flow up hill.

If for any reason you can not install your backflow at the top of the hill ABOVE the entire irrigation system, then you need to use a different backflow device. There are other devices that can be installed below the irrigation system.

Here is a link that might be useful: Irrigation Tutorial