Wurttemberg stayed loyal until after Leipzig and her troops retreated from Leipzig with the French, only leaving after their king summoned them home because Wurttemberg was being overrun by allied units. Nothing was done to the king of Wurttemberg.
I submit what was done to both Saxony itself and her king was because of the Prussian desire to ingest the entire country into Prussia. Fortunately, she was only allowed forty percent of the country, its population and army. It was still wrong and was merely territorial aggrandizement to Prussia's benefit, certainly not Saxony's.