I think the rule is in place ... just in case. It’s not really that big of an issue. If you have standard sized tanks, do a tank exchange. Otherwise, it’s 12 years for the tanks manufacture date to need recertification and every 5 years, thereafter.
Also, it means you don’t have to find out on the road that all of your propane leaked out or worse catch fire/blow up. Now, most of our TCs have propane detectors, so for the most part, you have it go off if you have a leak to the inside of the TC.
As far as side businesses are concerned, they may or may not check. I had a local place fill without and within a week, had a Love’s truck stop point it out. I actually didn’t know they had expired. Turns out my tank manufacture date was before the TC manufacture date. If they do notice and refuse to refill and you are on the road, it would be an extreme hassle.