There is no blanket reason here, and Propublica gives none.
All law enforcement officers can arrest citizens who interfere with or assault them.  If 130 Americans, including elected officials, were arrested and accused of this, okay, so?  The law enforcement officer can arrest them for that.  It is up to the prosecutor and courts what comes next.  Tossing the charges after the arrest doesn't mean the officer acted improperly, just that the prosecutor decided for whatever reason not to pursue the charges.  That happens all the time.
Again, so?  People get arrested and later released without charges all the time for lots of stuff.  Why is this any different?