He thinks "I make a billion shares. I sell one for 1BTC. Now I has one billion BTC worth of shares." The problem only exists for people who think that way. Reality is you have a billion shares that no one is buying and 1BTC.
Oh. Duh. Yeah, the shares are only worth what other people are willing to pay for them. For the same reason, you cannot calculate the value of a large amount of bitcoin based on the current exchange rate, because the act of selling that large amount would change the exchange rate.