Local rates seem reasonable if the local municipality/state/Gov't subsidies the courses, e.g Torrey Pines etc, and it is therefore reasonable to return some favours to local taxpayers. Perhaps the same argument.
Having said that, I have never seen a UK/Ireland course, many of which get local funding, offer the same discount to locals, at least not officially. I am sure there are some that do, but it must be an uncommon practice. The reality must be that these are simply ways of squeezing more money when there are adequate numbers willing to pay. And if you reduce visitor numbers as a result, perhaps that is just an added bonus.