Boots, the products I am referring to are the most popular in the residential landscape market not commercial "injectors". All of which do not have pumps or produce additional pressure in the line. For example, Fertile Earth, Fertigator, Ez Flo, Strong, etc... The back flow laws have been a serious issue for the fertigation industry and I have personally delt with this issue in over 40 states. Even within a state, the laws can vary, that is why I suggested checking the local codes first. All in all, the majority of these counties require a PVB, not an RPZ. This may not be true for your area, but once again, it differs between counties.
The common misconception is that these systems are injectors. When reviewed by USC, they were classified as proportioners due to the fact that they do not increase the pressure within the irrigation line. However, a commercial injector, such as turf feeding systems, will increase the line pressure through the use of pumps. These systems will require an RPZ in any state or county. I do not think this consumer was interested in a system that costs several thousand dollars.