Obama is only in office ~ 3 more years so unless he gets an amnesty bill that gives them a green like card they are taking a risk signing up.
Of course Obama wants them to sign up.
Of course a GOP POTUS is unlikely to try to throw them all out. But they dont know. We are citizens.
There is no risk. Once legalized, they will eventually be given a green card and citizenship. Cantor, Goodlatte, Ryan, and Issa are working on that now. You can't unring a bell as we are finding out with Obamacare.