I imagine (aside from the 'eh, didn't think of it/assumed we could just make it your problem' logic) that there are some notrivial issues:
Some readers (I certainly haven't played with them all, probably depends on IC and power budget) only register one card if more than one is simultaneously presented. Exactly which one it's going to be seems to be up to the dark, unknowable, gods of real-world RF behavior; but you only get one and no indication that there are others.
If the one you do get is valid for the purpose(and as best I read it, that can be either a TFL-issued card or a bank-issued payment card, at least on some of their systems) there isn't much to go on. You could cut the power, ping again, repeat, and trade time for certainty; but users aren't going to wait for that while boarding(or necessarily know whether the right answer has already been hit, or whether they are about to be saved from the wrong answer).
Assuming that they do have units that read multiple cards simultaneously, you'd think that the answer would be easier: Do they have some sort of pre-paid weekly/monthly pass? Use that first. Credit on a card issued by us? Second. 3rd party payment card, 3rd.
I imagine that they either don't have readers good for that, or they don't want to learn how creative some snarky member of the public gets about structuring the presentation of multiple RFID cards to create favorable misinterpretations. (Something that has already been explored... Who ever would have imagined that a hedge fund guy would bring amoral value-rationality to financial matters?)