Hiring migrant labor is sort of weird and frustrating. I'm not convinced that a one-year regional shortage is due to any particular regulation. Sometimes weather oddities (like this year) mean, for example, that the crews are still in California working almonds or something when they would ordinarily be moving north to work cherries. Some years, it's just insane, having a whole years income out getting overripe in the fields and not being able to find anyone to help harvest.
It's also true that hiring teens, or anyone else who's inexperienced, for farm work is sort of a loss leader. In our state, harvest workers are usually paid at piece-rate, and it's usually set so that good workers can make $10 - $14 an hour. Green teens, if they even show up, earn about $3 an hour by this method, so the state makes farmers make up the difference between that and minimum wage. This not only is economically unfeasible, but it really pisses off the guys who are working hard. (They think the kids are getting paid extra because they're gringos.) If you can convince them to come back every year, most people get into the rhythm.
The long-term trend is to use mechanization to make more steady (rather than seasonal) jobs for fewer people. But there are still a huge number of crops for which there is not yet any mechanical solution.