backflow preventer

Hello; I just replaced the garden hose faucet on the side of my neighbor's house. The old faucet had a (cheap DIY store type) backflow preventer installed. This is the first time I've ever seen one on any everyday outdoor faucet, so I did some research to find out why they might be required. So...

My house water is from a private well. When I was changing my whole-house sediment filter, the relief valve on the water heater popped. I guess this is due to the fact that I closed the valves on either side of the filter housing to swap out the filter, and pressure that would normally be...