The government has a working system in place to keep track of Chinese spending overseas. No doubt they could re-jigger it and deal with foreigners but it's kind of naive of how bureaucracies work to think this is a simple matter.
It's such a small subset that they don't care. Foreigners with China issued union pay debit or credit cards can use those abroad with no issues.
What's funny is I can use my union pay credit and debit cards abroad but cant use the union pay app abroad to pay for things.