I've owned several houses in California and not one has had an anti siphon valve on it. A quick check around my neighborhood shows that non of my neighbors do either (at least on their front yard spigots).

I wanted to run a tap with flow backwards and found that all outside taps in both big box hardware stores near me had a valve in them. There was no way to tell this externally.

Sorry but this is not correct. The anti-siphon valve is almost always apparent. These spigot valves are not very good and fail pretty regularly so I would not trust my drinking water to them.

Also an anti-siphon valve only works if it's above the water lines. If you store a hose above the spigot like many people do, you've just defeated the valve.

Every outside tap here in New Zealand is required to have a back flow preventer (is this a different thing?). They all passed the recent test they had which was required by the local council and they are all pretty old. The 'check valve' is required to be replaced every two years as it can't be tested according to the documentation here. I don't known what this is and don't have one.

And they adjusted for low water pressure by poking a few more holes for each plant nearer the end of the the poly tube. By avoiding "nozzles" on their tubing - which would require pressure to operate -- they greatly simplified the design of their irrigation system.

Again anything that moves liquid from point A to point B requires water pressure or the liquid will not move. So the system you describe still requires water pressure to operate. I really don't see how this simplifies because you have to empirically determine the number of holes at each plant. If you have thousands of plants, I just don't see how this is efficient or scales. Maybe I'm missing something. For small numbers of plants maybe it can make sense. Driving through Watsonville, Ca today on my way home from the beach I saw hundreds of rows of crops growing with traditional drip lines.

Go visit the UCSC agroecology center farm. They have been using variations of this technique for years. I was surprised when I saw it too - I'd thought that it would need to be more complicated.

Wouldn't you have to replace sections of tube each season with this approach?

That's true - I asked about that and they said that they could sometimes reuse the tube if they were planting in a similar configuration - otherwise they just discarded the tubing (it is very cheap - another advantage of their system.)