You missed a couple of points.
Protest was declared illegal.
The protesters were granted time to disperse in compliance with the lawfully issued order before any further action would be taken.
The protesters had the opportunity to challenge that declaration.
The protesters declined the opportunity to challenge that declaration.
The protesters were informed what would happen if they did not comply with the lawfully issued order and granted time to disperse in compliance with the lawfully issued order before any further action would be taken.
The protesters chose not to comply with the lawfully issued order when they were in full knowledge of what would happen if they did not comply with the lawfully issued order which they had not challenged.
(Knowing what the consequences of the actions which they chose to take ) [t]hey should have decamped (unless they were willing to accept the consequences of their own actions).