Why not pardon undocumented immigrants?

Gregory Koger asks at The Monkey Cage why President Obama doesn't just pardon undocumented immigrants.

As a scholar, I am interested in the political actions that don’t happen, and this strikes me as an interesting case of non-action. One explanation is that I misunderstand the scope of the pardon power and immigration is outside that scope. Or, perhaps the President is reluctant to intrude on Congress’s authority to (not) act on immigration, although that seems unlikely since the White House’s “We Can’t Wait” strategy is predicated on direct presidential action to combat legislative paralysis. So, I welcome comments to correct my interpretation of the law or the politics of immigration.

It's a really interesting question, and one I hadn't thought of. As I wrote as a comment there, I think the answer is that pardons only apply to federal criminal cases, and immigration violations are civil offenses. From DOJ's website:

Under the Constitution, the President’s clemency power extends only to federal criminal offenses. Executive clemency may take several forms, including pardon, commutation of sentence, remission of fine or restitution, or reprieve.

Otherwise this could've fostered an even more inflamed political debate than the one we have now.