Those methods were commonly used. Also, I wouldn't confuse stupidity with ignorance. We, in modern times, are blessed with a level of understanding of economics that is unprecedented. While it might seem natural to us to avoid such methods, as they are (in some ways) self-defeating, medieval rulers were not so enlightened. Furthermore,these practices weren't entirely a means of acquiring finances; they were also intended to impoverish the lower classes, in order to make them easier to control.