The Olympics are over seen by the UCI and as a general rule most USAC rules default to UCI rules.

Having raced dozens of mountain bike races including two national championships I have never been questioned by an official for my barends. I think today was the case of an insecure local official. He could have given the guy a warning to take them off next time, there is room in the rule book for this-

USA Cycling and any Race Director or sponsor, and their
respective agents, officials, employees and volunteers, shall
not be liable for any damages or injuries arising from or
connected in any way with the condition or adequacy of any
rider's competition equipment or uniform, regardless of
whether or not such competition equipment or uniform was
examined or was not determined to be in violation of the
rules

And to, "If not allowing bar ends keeps you away from a race, then whatever." The guy had Ergon grips that the bar ends are an integral part of the grip. He told me he trained hard and it was his first race. If an official belittled me and was a **** after I paid money to race, I wouldn't come back. Is this what we want?