The higher TA recommendations largely has to do with assumptions that everyone is chlorinating with trichlor, it is highly acidic and a pool needs a higher TA to keep the pH stable. I guarantee most pool store employees don't understand this and are just parroting the number they were told. For pools using liquid chlorine or saltwater chlorination the TA needs to be much lower to keep the pH stable.
As for the convenience of pool store testing, well that I just don't get. Even if someone confirmed that their store was 100% accurate (rarer than a chupacabra riding a unicorn) you still need to go there. Even running a full batch of tests on a TF-100/k-2006 takes 5 minutes, and a full batch is only really needed about once a month. The pool store involves taking the sample, driving to the store, waiting in line, allowing them to run the test, telling them no to all the recommendations, and driving home. Even if you are already going to the pool store for something else it doesn't take any less time, so what is more convenient? And again, that is if the store testing is accurate. Since we are talking the same people who don't actually understand what TA does in a pool (saying it turns hair green is hilarious) I would be pretty hard pressed to trust their testing abilities.