Originally Posted by tubemonkey
When data plans are unlimited, charging extra for tethering is justified. But when data is a fixed amount, it's robbery to charge for tethering.
No; the reason for it is that if you have tethering, you're far more likely to use your allowance than if you don't. Eg, my Vodafone contract on my iPhone gives me 1GB of data per month. I never come close to it, despite using my phone pretty much all day for Internet access; my typical usage is perhaps 200-300MB in a month. If I had tethering (for which I'd have to pay £5/month more) I'm certain that I would use all my data allowance every month.