I believe you are wrong here.
In your example you can actually watch the person, you can even videotape (or digitally ofcoruce) him/her.
But, in the case of your phone or pc, the supplier/isp refuses to tell us exactly how they count and what they count.
For example, phone service providers will often charge differently for a picture message than for data traffic. In which case they may or may not deduct the snooped message sent via IP from your data total and add a fixed number to the bill instead. On the other hand they may later switch to a unlimited plan where they now only bill for the data, but will not clarify, or even know themselves, if they are now still deducting the picture messages.
In comcast case they used an independent party to verify their billing and then went out and stated that they felt it was really good and only -0.76% to +0.36% wrong. The problem there is any error in such sales shall be on the side of the consumer, so they would have to offset all their billing by +0.76% to be in the clear from a regulatory perspective.