"The state ought to be providing upkeep on our behalf for historic buildings and ancient sites out of the taxes we pay"

I agree. In a perfect world, the NT's properties would be like our national galleries - subsidised by our taxes and free at the point of use.

To be fair to the shops, they are just another way to generate revenue - they have to be as easily marketable as possible to the typical NT visitor, and I assume they generally do pretty well and cosnequently serve their purpose.