Explanation: Mayors are taking steps intended to result in a Democratic President. All else falls under that strategic objective.
Lockdowns (and social distancing, as part of that structure) encourage mail-in elections, which are more easily compromised by fraud.
Riots encourage social unrest and divisions, intended to discourage blacks from voting for the incumbent party in the presidential election.
So, enforcing lockdown or social distancing rules on rioters would subvert the actual purpose of the lockdown, by interfering with another part of the strategy.