And even if they tried, there is no way to prevent users from just across the line in Alabama from searching for prostitutes in a bigger city in Georgia (geolocation based on IP address is not always that accurate).
But let's say the site could someone manage to ensure that no interstate commerce was going on between users--there is still interstate commerce happening because the company is operating across state lines by paying for all those servers/generating add revenue from all the users.
Furthermore, did you read the rest of my comment? None of the above really matters because a bunch of sites engaging in intrastate commerce can still have a substantial cumulative effect on interstate commerce.
Why is that so clear?
It's fairly easy to create an infrastructure that directs users to servers solely in their state. Prostitution is a fairly local affair.