It makes perfect sense. Those who place a high priority on anti-immigration would not support Jeb anyway. He is there to suck up (conservative leaning) votes from candidates other than Jeb. He doesn't have a chance in hell of getting elected and he will likely drop out once enough candidates have been eliminated. Just like Newt.
Second, it's not just the illegals message, but it's the way Trump is delivering it, as "in your face," challenging the media. THIS is what people are responding to as much as the specific issue. Cruz has done this some. Paul has cow-towed. Most of the rest have just slinked away under the first media attack. Where was Rubio when the Times tried to slime him with the boat? Trump would have turned that against the media, and Hillary.
Do not confused Trump's position on issues with his very apparent appeal to conservative voters who are desperate for ANYONE on our side to aggressively fight back.