There is more than one way to forage. At best, the army can pay the local inhabitants, maybe even a fair value, particularly if the area is nominally friendly. Even if the army "commandeers" supplies, it would want to keep the criminal activity to a minimum, if only because it is bad for discipline.

The Thirty Years' War is an extreme case, in large part because the armies were made up of mercenaries that were not getting paid regularly.

Unhappy is being used here to mean "unhappy enough that it affects you." It's not about concern for the mental health of the locals.