AlvinofDiaspar
Moderator
The same thing that stops you simply walking out of a grocer store with a loaf of bread without paying.
Personally, I've never observed anyone standing at a transfer machine getting 12 transfers before. Do you observe this happening often?
Let's not pick on hyperbole though - the fact of the matter is fare evasion under the current (soon to be replaced, thankfully) system is incredibly easy, with the whole process from payment to verification requiring way too much human involvement while producing subpar results.
AoD