While the revelation came first as a “greeting; not by commandment or constraint” (D&C 89:2), when members of the Church had had time to be taught the import of the revelation, succeeding Presidents of the Church declared it to be a commandment. And it was accepted by the Church as such.
Key point is that it is for members only...so for all you non-lds BYU fans, I'll cheer the cougs with you while you enjoy a frosty adult beverage.