This is pretty scary, I was recently on both of these (currently I’m on Cephalexin) but didn’t experience the tendon soreness that leads to the ruptured tendon. My doctor didn’t even warn me that this was even an issue, so it’s probably a good idea that they add it to the label to let the user know.

The FDA’s action came after the consumer group Public Citizen petitioned_and later sued_the agency for such warnings. Regulators took too long to act, complained Sidney Wolfe, head of the consumer group’s health section. Many injuries “would have been prevented if patients and doctors had known a pain in the tendon is an early sign that leads to rupture,” Wolfe said. Public Citizen’s original petition was filed nearly two years ago.
FDA officials pointed out that prescribing literature for the drug class already carried clear warnings of the risk of tendon rupture. They said the agency acted to emphasize the warnings because continued reports of injuries indicated that the message may not have gotten through to doctors and patients.

Who reads the prescribing literature? It’s printed in tiny print that that you can’t read unless you use a magnifying glass and it’s written in medical jargon. And there’s usually such a long list of potential problems that a tendon rupture would be lost in all the data you would have to remember.
No, for big problems that are totally unrelated to the reason you’re taking the medications, they need it on the label.