Customers could have sold their BTC for USD at any time and just held USD until after the fork. There isn't any functional difference between holding USD pre-fork or holding both BTC and BCH post-fork.

That's wrong. If I hold 1 BTC, I'll have 1 BTC and 1 BCH after the fork. If I sell for the equivalent of 1 BTC in USD (say, $2500), I'll have $2500 after the fork, instead of $2500+$700 (price of 1 BTC + price of 1 BCH)