You just got through saying that it hasn't been ruled on by the SC so how do you know it's not unconstitutional? By precedent?
exercising discretion in who to prosecute and who not to prosecute is not unconstitutional.
you said 15,000,000 illegal immigrants here. how feasible do you think it is to track down, arrest, jail, prosecute and deport every single one of them? how long would all the courts in the country be tied up doing that? how many immigration officers would it take? how much money would have to be spent housing all these people in holding facilities & flying them back to other countries?
so where do you start? which people do you round up first?
would you start with people who were brought here against their will when they were less than 10 years old, who have jobs, pay taxes, and aren't otherwise in any trouble with the law?
or would you start with people who are criminals, who don't have jobs, who knew they were breaking the laws to come here and did so anyway, on their own, uncoerced?
that's what DACA is about. it's not a "law" granting amnesty to anyone. it's not giving people citizenship. it's not giving them SS checks or medicare. it's showing mercy, in an organized way, to a class of illegal immigrants that is the least offensive subset of illegal immigrants, by forgiving their trespass while the limited resources that are available to curb immigration are concentrated on the most offensive cases.