But presumably they do care about rampaging Muggles on witch-hunts?

That hasn't happened for about three hundred years. But then, by a mysterious coincidence, neither has torturing little children into insanity to appease the blood lust of the "nobility", something which is likely to get those pitchforks sharpened pretty damned fast.

"She is too young! Her mind would not withstand it! Not in three centuries has such a thing been done in Britain!"

Secrecy about magic does seem to matter to these folks, otherwise why go to all the effort? Possibly because in an all-out war the wizards risk losing. They have less magic now, the Muggles have much nastier weapons, and not all wizards would fight on the same side. The magical world would be itself deeply divided if torturing a child proved to be the causus belli. Quirrell for one thinks they'd lose (Chapter 34):

"Your parents nearly lost against half a hundred, who thought to take this country alive! How quickly would they have been crushed by a foe more numerous than they, a foe that cared for nothing but their destruction?...

And if some still greater enemy rose against us in a war of extermination, then only a united magical world could survive."