its not strange at all. If you go to the supermarket and you buy on 0,5l coke you pay 1,10€ if you buy 1,0l coke you buy 1,25€ if you buy 2,0l coke you pay 1,49€.
you always pay a minimum price (marginal costs) and then x amount for additional service
Yeah, and in this casse one could think of that 25 dollars as something to ensure that the person is actually a real, motivated streamer and not just some guy trolling around for 1 or 2 friends. For example, its more likely that people take promoting the tournament more seriously and actually follow the streaming rules (no flaming, racism, whatever) if they got at least some money involved.