If someone is starving in Mexico and they can come to the US to get a job and food, they will. That's a problem related to economics, not lawbreaking. It's like P2P internet downloads or prohibition, it's just going to happen if you create trade policies that set up incentives for it to happen.
If Obama is not responsible for the comments of David Geffen, why is Hillary Clinton responsible for the comments of Chris Lehane?
This is actually a good question. David Geffen is a billionaire supporter of Obama who formerly allied himself with Clinton. His words are his own, and he was not putting out message for Obama to coordinate a messaging operation.
Lehane is an unofficial surrogate of the Clinton campaign, a professional campaign operative who will benefit financially and politically via a Clinton victory. He is pretending to be a neutral observer of politics, and reporters are using him to give a Pravda-esque doublespeak to both readers and insiders.
This is a problem with honesty in political dialogue, a way the voters are deceived through cozy insider relationships with reporters.