Yes, if riots are not covered, they're not covered, but that will generally be obvious to the purchaser of the insurance (specifically by virtue of a clause about riots being excluded being present in the contract, in contravention to your claim above that "the clause [about riots] won't be in any contract to begin with". It will be there as an explicit exclusion.).

If the purchaser of the insurance makes the choice to purchase insurance which obviously and explicitly does not cover riots, they ought not be surprised when riots are not covered and it's not the insurer's fault.