Ultimately, you're paying fees in your starbase because that's the balance tradeoff for unlimited slots for everyone. If we take slots away we need some form of substitute (pseudo-)scarcity so that everyone in the universe doesn't just build in Jita 4-4, which would be bad for various reasons but primarily because it removes a whole lot of interesting decisions and makes the rest of the map an industrial wasteland, which is not a thing we want. We felt that scaling fees were a good solution to this, as fees were already a thing (even though they were essentially irrelevant) and because ISK is kinda the nexus of decision-making for serious industrialists. If we're going to use fees, we pretty much have to apply them everywhere, including starbases because they scale too well otherwise and we don't want to re-add pseudo-slots to stop them from being too powerful.