I think this is also an archaic model from before smart phones and the early days of smart phones. In the early days of apps, most attempted to limit data usage because most network providers charged a premium for data and the networks were much slower and smaller.
While you could tether in these early days, even before smart phones, the computer was capable of much higher data usage than the phone. These limits were put in place to protect a network that wasn’t really built for this level of load.
Old rules with good purpose turned into a way to charge more money.