1) Just because the work is hourly doesn't mean it's full-time and not seasonal.  Lots of people work at an hourly rate and are told not to come in if there isn't work to be done.  Most pickers have to move from one farm to the next to follow the work.

2) There is likely to be more oversight and pushing of employees when the work is hourly than when it is by the pound.  The employees have a great incentive to work quickly.  If someone doesn't take a break, it is largely by choice.  They can always work less and get paid less.  In this case, they just get paid less and don't have an option to earn more.