Here's a metaphor: Imagine if the water company charged you per gallon for water you used, but then added an additional charge for having a shower. Since you own plumbing fixtures to which the shower connects, and pay for every gallon, we would consider it unfair for the water company to charge extra for an "authorized" shower.
As far as theft of service, what on earth have you stolen? You pay for the data you transfer. Tethering is simply an "unauthorized" (by the vendor) use of that data.