I have no idea where you live where they are required by law, but I've never heard of such a thing. They've never been required by law anywhere I've owned a car.
In any case, given they have batteries that eventually wear out, and older cars don't have them, it would still be more effective to use a camera.
Edit: even if they're required in new cars that doesn't make them mandatory on the road. It's not like all tires must be sold with TPMS.
The TPMS requirement in the US is ratcheted. Once you have TPMS on a car, it needs to stay on the car even if you replace the wheels. It must not be disabled or removed, and it must be fixed if broken.
Obviously, it is easy to flout these regulations as an individual or as a small-time mechanic. But there can be penalties if tampering or negligence gets discovered, the most common of which is failing to pass inspection. When I bought wheels from Tire Rack online in 2015, I had to either pay extra to have them equipped with TPMS or else attest that my vehicle never had TPMS.
The real purpose has nothing to do with surveillance but with consumer and environmental protection. If you buy a used car with TPMS, you should be confident that it works. And keeping it working increases the chances that people will keep their tires filled and their fuel economy up.
However, a new problem has arisen, which is that gas stations don't bother to fix their broken air compressors anymore.
I can find no mention of a TPMS requirement in my state. My older car has a malfunction in its TPMS and has for 10+ years. The repair cost I was quoted was ridiculous and I just periodically check the tire pressure with a normal gauge.
This vehicle has passed all required inspections for that entire time and the TPMS system hasn't even been mentioned in that context.
edit: I found the claim that TPMS was required interesting and a quick web search shows that 13 states in the US don't require vehicle safety inspections AT ALL.
It is not a state law, it is a Federal regulation under 49 CFR ยง 571.138 [1]. These requirements were fully effective for manufacturers and "alterers" as of 1 Sep 2008. However, yes, if your state doesn't require it (or doesn't require any inspection), then there's no way to penalize you as vehicle owner or reseller. Mechanics and aftermarket part sellers can still run afoul of USDOT though.
In any case, given they have batteries that eventually wear out, and older cars don't have them, it would still be more effective to use a camera.
Edit: even if they're required in new cars that doesn't make them mandatory on the road. It's not like all tires must be sold with TPMS.