Context is North America (Canada). Water heaters are always equipped with a pressure/temperature safety relief valve (no argument that safety precautions are a good thing). My question is: how is it possible for the water heater to build up pressure when any expansion could simply drive the small amount of water backward into the incoming supply? Is there a check valve somewhere to prevent back flow? So far as I know, there is no such thing built into the water heater... am I wrong? Is there one built into the water meter?
Either an actual check valve or something that acts like one (such as a Pressure Reducing Valve) are fairly common on cold water supplies.
The short answer is that when you heat cold water it will expand. Even without bringing it to a boil. It's about 2% in volume from 20°C to 70°C.
The typical installation of a boiler has a backflow preventer on the feed side. So that when you open the cold faucet you don't suddenly get hot water.
Those safety valves are to relieve pressure by dumping the contents into the surrounding area when it starts boiling to avoid a rupture. These contents are boiling hot and not something you want to deal with often. Instead there is an expansion tank to absorb the expected growth in volume without letting the pressure increase too much and possibly trigger a dump or damage the fittings.
Historically the most common arrangement was a tank and pump in the basement with a foot valve in the well. Even with a deep well or submerged pump you'd want a check valve to prevent the water tank from raining back into the well. Long story short, no backing up into wells.
Let's go to the other extreme and consider municipal supplies, which largely depend on gravity and towers for pressure. A 200' drop gives you a cozy 86.8 psi, which is why pressure reducing valves are common. Not to mention cities that require backflow prevention to keep the water supply uncontaminated. You really want the pressure relief valve in the city too, not just the country.
You have overlooked one other thing though. A "small amount of water" is a large amount of steam. If a plumbing problem left the water heater half-empty you'd have a boiler rather than a heater and the expansion would no longer be slow or minor.
Safety aside, there's also something to be said for designing a particular point of failure into any system. PEX, I believe, is rated for 80 PSI at 200F which makes it a relatively poor option for carrying pressurized hot water back through the "cold" supply. Even with copper pipes you'd probably rather have the relief valve open than have your cold water loop find its own relief valve, such as the icemaker. Basically the relief valve opening is much less of a mess than a random pipe or fitting on the supply side failing. On top of that the relief valve will close once the pressure decreases again, where as the broken pipe will just keep filling up your house with city water pressure.