The way I make sense of that is to theorize that Bretonnia blows right past the standard Feudal manorialism and into an outright palace economy. A peasant gives up 90% of his harvest, but then the lord redistributes most of what comes in as well as a large amount of meat hunted by the otherwise idle nobility, resulting in a peasant class that actually has a healthier diet than if they just ate whatever grew well on their patch of land. It's a system that would be incredibly vulnerable to abuse, but even those nobles who don't internalize the tenets of chivalry and the commandments of the Lady might think twice about abusing their power, because Bretonnia is absolutely crawling with Knights Errant who would come running at the slightest hint of a rumour of a corrupt lord abusing the peasantry. Especially if the Damsels play a part in keeping the system running as it should because their movements can't be impeded by the nobility and they're trusted implicitly by the peasants, so they'd be very able to hear about any abuses and can get the wheels turning on correcting things.
6th Edition describes a society that should fall apart within fifteen years, yet has lasted fifteen centuries. The palace economy thing isn't a justification, it's the way to be as faithful as possible to the canon material without having to suppress your common sense.
