Police swear an oath to uphold the law and it’s ostensibly their job to, when observing illegality, say “hey knock it off.”
Protesters do not swear an oath as a prerequisite to protesting. Protesters frequently do speak out against- and even forcibly prevent- looting and vandalism within their ranks, but if they don’t, it’s not a dereliction of duty in the same way as when cops ignore abuses in their ranks.