Because when it is in a casino, the machine is hooked up to a central database that keeps track of the tickets issued anywhere inside the casino. When you insert a ticket into a machine, the machine connects to the central server database and looks up the ticket, if it is found, then the machine is authorized to give as many credits as the server says the ticket has. Once you remove the machine from the casino, it no longer has a place to validate the ticket, so it just times out and spits it back out. At home, the central server is replaced by the Bettor TITO device, but it doesn't store the ticket value, rather it prints a barcode with the credit value on it, so when you insert the ticket, the TITO board will interpret the barcode credit amount and authorize the machine to issue the credits.