Dumbledore never promised to stop reading student's minds. Not int chapter 18 when he said Snape would make that promise or in chapter 20 when he is called on reading HJPEV's mind.

Also, Dumbledore's offered compromise to HJPEV was this:

He will promise to only read minds when the safety of a student requires it.

It is not difficult to argue that the safety of some student, somewhere requires constant readings of the minds of the Weasley twins.