You're right, my use of green card is incorrect -- it's visa. Green cards do expire, though, after 10 years if I'm remembering correctly (it almost happened to me).
I just looked up DACA, thanks for pointing that out. Am I misreading it on Wikipedia?[0] It doesn't seem very discretionary from that second paragraph of requirements and guidelines. That seems pretty concrete.
[0] https://en.wikipedia.org/wiki/Deferred_Action_for_Childhood_...
Repealing the ACA and cutting taxes is pretty well in keeping with stated Republican goals (one of the stopping blocks they have for their repeal is the group of conservatives that think it leaves too much government in the insurance system).
DACA is discretionary in the sense that the government has information that the registrants are not following a regular immigration process and still is not deporting them. "Deferred action" is discretionary enforcement.
Not deferring action is also discretionary enforcement, it's just a different choice on how to apply the same discretion.
https://www.irs.gov/affordable-care-act/employers/questions-...
It's more expensive to have 50 employees than 49.
Also, a green card doesn't really expire, people overstay travel and work permits. A green card is for immigrants that have permanent permission to stay.
Immigration policy certainly has lots of discretionary enforcement. DACA is a big example. I think it's reasonable and the right thing to do, but it is discretionary enforcement of immigration law.