After working in various customer service jobs and hearing from countless people who had screwed up and wanted me to move heaven and earth to help them or were outiright lying about stuff and would go on about their 'major medical illness' I tell you my level of sympathy got pretty low. When it comes time to pay for their mistakes everyone has a sob story or some twisted logic to offer up.
Was this business unnecessarily rude? Only the op and the people they talked to knows for sure. Should the renter have been more responsible in handling their reservation? Undoubtedly. Only lesson I learned from this thread is make sure the details on your res are correct and if you are going to keep the car longer check with the company. Which I think I already knew.

If the choice is cheapest rates and risking this kind of treatment if I screw up my res like this, I'm fine with that.