Many retailers still sync up daily via batch processes creating a race condition where copies of a real receipt could be used at multiple locations on the same day.
There are also issues with exception handling processes that dictate what clerks are to do if there's a problem with a receipt that are likely exploitable if photo receipts are accepted.
As it stands these holes exist but the harm is reduced by the fact that if you want a fake receipt, you need the right receipt paper and printing equipment which basically means that only professional criminals have a shot at exploiting the problem.
There are also issues with exception handling processes that dictate what clerks are to do if there's a problem with a receipt that are likely exploitable if photo receipts are accepted.
As it stands these holes exist but the harm is reduced by the fact that if you want a fake receipt, you need the right receipt paper and printing equipment which basically means that only professional criminals have a shot at exploiting the problem.