Couldn't shell companies that had no direct relationship with bitcoins in theory be used, if that were the case?
I suppose, but that seems (I don't actually know) like a lot more trouble than just not dealing with bitcoins and having a bank account.
A large number of bitcoin businesses are exchanges of some sort - ones where people are looking to exchange dollars to/from BTC. Going BTC all the way down is going to take a long time.
Less trouble than convincing the state and utility companies to accept bitcoin settlement for taxes and bills.