I'm not saying that you need laws to make the world better, just that some of them do.

This belongs in that category in my opinion because it is something that costs very little in absolute terms (manufacturer has to run some tests and print some numbers that they probably already had), but it makes the whole system work better because it enables people to vote with their wallet, and gets inefficient products eliminated because people can spot them before sale.

No company would advertise a "20% below average battery lifetime" without regulation like this, which is why objectively bad devices can still get sold easily on unregulated markets.