Some people seem to be justifying the way mobile networks try to discriminate against tethering. One comment even suggests that multiple devices is a valid justification. Really? Isn't it just as simple as measuring the actual volume of megabytes used, regardless of how many devices are connected?
Lets consider a couple of analogies:
Of course, these ideas sound a bit outlandish but the only difference between these crazy ideas and tethering charges is that mobile phone networks usually have the monopoly/duopoly to get away with it.
Mobile networks actually dread the idea of becoming commodities like fuel. They thrive on price discrimination and they seem to have an endless number of tricks to get extra money out of people who may have it by deliberately crippling parts of the service for everybody else. This goes against the ideals of innovation and efficiency and in the most extreme cases their practices are widely recognised for the sham that they are.
Revealing that you have connected a laptop to your phone may be like revealing you live in an affluent area or that you have an expensive car. Plenty of businesses try to exploit such knowledge when deciding how much they can charge you or other aspects of how they provide you with a service. This is why privacy is important in every feature of the technology we use.
This rogue behaviour by "creative" marketing men also helps to understand just what would happen in the world of technology if it wasn't for the use of free licenses, open standards, accessible encryption and net neutrality.