I'll tell you the reason. With that kind of mileage you are getting close to the end of the transmssions' life. If the shop does the flush and then a few thousand miles later the transmission burns up then there is the appearance that the shop messed up the transmission by doing that. At the very least you end up with a mad customer blaming you for the problem and at the worst you end up a lawsuit. Most shops are like that. I know personally I don't recommend it due to the perceived liability.
