Who said they would check all address changes? Customs don't check all travellers - you fill out a form and then sometimes you are stopped perhaps randomly or perhaps for a reason. No doubt UA will do similarly. You provide the paperwork and only sometimes will they check it thoroughly.
In reality, any abuse will be limited to those under the PQD threshold who either would be 1K, or who would be a lower tier and don't have the credit card (which in theory is only available to US residents). So, once you've boiled the numbers down to that small set, and only look at those who change their address from the US, and screen out those whose trips generally originate outside the US - and then only sample say 1 in 100 - you will have a small number to deal with.
All UA has to do is find two or three culprits and publicise their expulsion from the program and they will have frightened off most of the abusive (largely FT) community.