"Was their treatment of prisoners out of sync with the standards of the time? "
Yes it was very out of sync. Most nations tried to take care of POW's as well as the situation allowed for.
In 1812 the Russian civilians boiled some of the French POW's alive, some were knouted to death, and so on, but for the most part the French were treated as well as possible even then.
An exception must be made for "franc tireurs" and other civilian uprisings against military power. Be it the Milan uprising, the Flemish boerenkrijg, the Luxemburg knupfelkrieg, the German "Rumpelhannes" (great movie with curd Jurgens in the lead); the different uprisings in Tyrol, and of course Spain. In all those cases the (French) repression was supposedly without mercy. "Supposedly" since it seems that the Young guard did not execute the Spanish caught in the Dos de Mayo uprising after all, Joubert refused to execute the Tyroleans caught after the battle of Brixen, the Spanish POW's were not treated as bagnards as they were supposed to, (though a great many died from malaria working in the marshes of Niot).