Wearing riot helmets and carrying zip ties, Boston police officers moved in one day this week and surrounded a group of pro-Palestinian protesters on a grassy patch of Northeastern University’s campus. Six police wagons were idling nearby, and an officer had issued a terse warning. Mass arrests looked imminent.

Then, without explanation, the riot police packed up and left.

The sudden end to the standoff produced cheers from the protesters, and confusion for those who had been bracing for…