Besides the fact that tokens cost money to make and they can jam the slots, the bill-acceptor machines use U.S. CURRENCY for input. Counterfeit that and you've got the FEDS on your back!! (Of course, in the old days, the guys that would be sent after the counterfeiters weren't too polite either!)

I assume that TITO's are pretty secure too, since they cross-reference the ticket as legit before cashing it.