That's possible, but not very likely. The green card is a legal permanent resident visa, which allows them to live permanently in the US. But there are some from Mexico and other nations who have a green card but use it on a temp basis. In some cases, it is easier to get the green card than it is to get a temp worker visa. Certainly, having a green card allows the worker to bypass yearly renewal of a temp visa.
The green card also allows them to enter the naturalization process whereby they become citizens. But not all green card holders become naturalized because they have to pay hefty fees, hire lawyers, and pass the test.
More likely your clients are using the H2A temporary ag worker visa which is seasonal, and requires the employer to provide housing.
For some farmers, the H2A works very well, but for most, it is a bureaucratic nightmare and the ag industry wants it reformed as part of comprehensive reform. There is an unlimited quota on H2A, but there are not very many H2A visas being utilized.
The other common temp/seasonal visa is the H2B Non-Ag worker visa which is used by the hospitality and lawn care industry.
Immigration reform would also create a third temp worker visa but it would be for a longer period such as 3 years.
Quite possible I used the wrong term for the program.
Thanks for the specifics—I don’t believe we need any of them.