Likely a negotiating ploy to get them to step up and do their share. Classic business negotiation tactic...until you threaten to walk, and you are called back to the table, you can't be sure you have the best deal possible. Totally in line with how he does business, and how most successful businessmen do business. Leaking it even makes the ploy more effective...unless his domestic opponents try to undermine the move by having a nuclear freakout.
But, I'm willing to talk about a withdrawal anyway. Over the course of this century, the US will become less and less capable of being a global superpower, and it is probably a good thing for Europe to defend themselves now. They can handle the Russians.