There's rhetoric that plays on violent imagery (like Palin's cross-hairs, or saying we're going to smack down our opponents in the next election), but if reasonable people wouldn't regard it as actually calling for violence, that's not even morally wrong, much less legally. That's why I defended Palin from attacks over that cross-hair imagery, all those years ago.
Picture two different scenarios:
(1) An NFL coach gives the team a pre-game speech where he says "we're going to war" and "let's kick their asses." Then, during the game, one of his players seriously injures the opposing QB with a dirty play.
(2) An NFL coach promises to pay any fines the league puts on any of his players for unnecessary roughness. He even puts a bounty on the opposing QB -- offering a bonus to anyone who knocks him out of the game,
Buddy Ryan style. Then, during the game, one of his players seriously injures the opposing QB with a dirty play.
In the first case, that kind of rhetoric is pretty standard in the NFL, and no reasonable person would have understood it as incentivizing efforts to injure opponents. In the latter case, the rhetoric is extraordinary, and crosses the line from metaphor/hyperbole to actually inciting criminal behavior.
Everything I've heard from Obama falls more in the first category. It's common-place rhetoric you could find from major politicians of both parties, which traffics in metaphors of violence. Nothing terribly interesting there. By comparison, when Trump promised to pay the legal bills of anyone who beats a heckler at one of his rallies, that's not something I'd ever heard before from a major politician. That was no longer a metaphoric reference to violence. It was a promise to protect people from some of the legal consequences of ACTUAL real-world violence.