To understand the system, it must be known that Metro tickets are valid for a limited period of time after they're posted. In a country where solidarity is an important value, it is not uncommon for travelers to let their still valid ticket on the machines that are used to validate them. Those in need can get it travel for free with a valid ticket.
And to reassure economists, it seems that the institution has taken the problem into account since its machines have been equipped with inclined tops, in order to make it impossible to let a ticket on. I hope the Daily Mail will be happy.