NFC can be programmed to work the way you describe because of the way NFC works. There is a secure chip in the reader that connects, activates the phone, asks for your approval and then transfers of information. The key is a specialized terinal that activates the phone. These are still rare.
For MST, there is nothing on the reader to activate Samsung Pay -- which is the absolute beauty of it. Of course they could have your phone constatantly broadcasting your info so it would just work when you put it up to the terminal . . . Clearly the wrong thing to do on so many levels. As a result, since the terminal cannot activate your phone, you need to.