Good question. I'm by no means a constitutional expert, but it appears there is NO law making prostitution illegal, so by default it is legal. In the past, the government created laws around the business transaction. So although the actual sex act was legal, the negotiation and business transaction were not.
In a way, it's the same as youth smoking. Stores can not sell to kids, but it is not illegal for kids to possess or smoke.
Also, the judgement was suspended for thirty days to allow the government time to appeal.