If a company hired and trained you but has not yet put you on the schedule, if you decide to quit can you demand that you be paid for your hours?

That depends. Were you a contract employee or an "at will" employee? If you were a contract employee it depends on the terms of your contract. You speak of the job as though it is an at will position (these are your typical hourly jobs). If that is the case you are entitled to get paid for the hours you trained. That new hires quit before they complete training or get put on the full-time schedule is part of the risk the company takes in hiring people.