Reasons for this should be down to the fact that you can't afford the object/service in question, thus making this phrase popular among students.
The phrase is coined from an observation of the Munich public transport system. The whole system relies on the system of trust, as conductors are few and far between. You should pay for a ticket, but it is easy too spend a week there, use the trams daily and not pay for one single ticket.
*down several drinks