I think the reason why they don't want their GPUs used for mining is because mining uses 100% of the GPU for 24 hours a day and can cause some components like capacitors to fail more often. Usually gamers play for 1-2 hours a day and the GPU doesn't go thru as many "useable hours" compared to mining.
That and those who dont bother underclocking... Also fans will wear out sooner. Lots of little reasons why they dont want us to.