The pizza purchase was not the first transaction to assign monetary value to BTC, by the time that happened (2010) it was already well established that BTC had monetary value.

People were already buying/selling for nonzero dollar values in 2009; lacking a true exchange with order book, the bid/ask was usually based (rightly or wrongly, you decide) around the marginal cost of generating new coins via mining.

2009 pricing history:
http://newlibertystandard.wetpaint.com/page/2009+Exchange+Rate