Carriers want to sell "unlimited" LD plans. Customers like unlimited LD plans. And for most of the US, LD is cheap enough that you can make that work. The model breaks down a bit if you have to start subsidizing your customers' calls to sex lines in quantity.
Because that would be a clear violation of he principle of carrier neutrality - the point of it is that anyone can connect to anyone at one price.
Stupid question: how do long distance charges fit into "connect to anyone at one price"?
You can charge differently based on how far the data travels, but not whom it travels between. Hence, the weird "kickbacks" for sex hotlines. If I dial 1-900-SEX-TALK, the long distance carrier can't demand a cut of the $2.99/minute I'll pay for that call.
Well, if different networks can charge different amounts for access, why not charge different amounts for different networks based on what they charge?