Some of them do so out of expediency, conservative evangelical Christianity is rapidly growing in Hispanic populations. But, some of their support is genuinely principled, and in opposition to their own Republican leadership and to many in their congregations.
Politics, like religion, is a tangled and messy business, contrary to all of the ideologues who would do so much more harm by trying to tidy it all up.
I am surprised, pleasantly, by the support of white evangelical leaders on this issue.
I'm not surprised by the growing evangelical movement among Hispanics. In my experience, these communities tend to be socially and culturally very conservative. They can be brutal on their gay children, especially on gay sons (a large percentage of gay runaways in New York is Latino). As the moral credibility of the Roman Catholic Church falls like a brick, the evangelical churches have been aggressive in recruiting among disillusioned formerly Catholic Hispanics. They have been so successful that there are now large Spanish speaking evangelical congregations with their own powerful and influential leaders. Those leaders cultivated friendships with older white leaders, and have successfully persuaded them to support humane immigration reform, and to oppose punitive and discriminatory measures like the Arizona law, often contrary to the wishes of their own congregations. They now find themselves on the same side as many Catholic, Mainline Protestant, and Jewish leaders on this issue.