Prep Conductor is available to companies with less than 100 users - you just need to purchase a min of 100.
So if your company has 5 seats on the server you pay the same as 100
It's simply a matter of Tableau deciding what minimum purchase is required to make the sale. It's sort of the same as going to the Costco store and they sell boxes of cookies with 4 packages inside. It's a company decision on pricing. There is no technical reason.