1) I'd rather lean towards Hanlon's razor 2) Obama was actually quite mindful of int'l law and seriously restricted aerial strikes 3) Once Trump came to power, the military has been allowed carte blanche, and they basically wiped Mosul and Raqqa to take ISIS.

As for your first paragraph, the point is that if you double-cross the enemy on a deal on made with them, then they won't make any more deals with you, and the U.S. needs leverage in Syria more than anyone else. So it's likely that the U.S. said, "you get out of Raqqa, we'll let you get to Albukamal or [whatever other place], and after that you're fair game again." Otherwise why would these jihadists have taken the bargain? They knew they would be a sitting target without assurances of safety until a destination.