I don't like charging for both either. But the best argument that I have heard on why they do it is that the pricing and tiered models and network support structure are based on an expected data throughput for a single device. Once you start adding devices connected through a single hotspot, the throughput increases significantly, thereby putting an additional load on the network that does not match up with the assumed/established pricing model. So, in an attempt to somewhat limit this activity (and keep throughput more manageable), a separate charge is a incurred for tethering. The counter-argument is that this really shouldn't happen with tiered pricing structures as the tiers (and their associated cost) should account for this activity.