It's 100% not their fault if someone else gets it wrong, it's true. But modern phones are complicated devices -- if you get your battery replaced poorly, and a month later your phone dies because the case was resealed incorrectly which compromised the water resistance and something got damaged as a result, is the consumer likely to think "oh, I bet that was a poor third party repair" or "ugh, iPhones suck, they're so unreliable"?
That said, I don't think this battery change is particularly tied to the personal repair issue. I think this is a shot at the resale / refurb market. Companies buying up old phones, "refreshing" them with crappy components, and selling them on to customers who don't realize that the phone is going to need a battery replacement in three months. In that case the customer didn't have the information available to tell that they'd been sold something shoddy, whereas now they can check the battery health and see this new warning.