Well, if your LAX-SVO flight only allows you one checked-in luggage then you should have indeed paid for the excess luggage.
You mention you had 2 separate tickets, was it AS ANC-LAX and SU LAX-SVO-XXX? Do you have extra luggage allowance on AS? If so then they made a mistake by allowing you to send them through since even if it was on one ticket, SU luggage allowance would have been the right one to follow.