The key difference is that the paper ticket specifies the entry and exit stations.
When tapping a smart card you don't specify the exit because that's exactly the thing that makes it a faster/more convenient method of payment. There is no way to check whether you intend to pay until you exit, and then it's already too late unless a physical barrier is present at every station.
When tapping a smart card you don't specify the exit because that's exactly the thing that makes it a faster/more convenient method of payment. There is no way to check whether you intend to pay until you exit, and then it's already too late unless a physical barrier is present at every station.