Intent or incompetence
I have an O2 Femtocell - named by O2 in priapistic pique the 'Boostbox'.
In their wisdom O2 count all service consumption (voice, sms or data) through the boost box as normal service consumption, as if the full radio access network (the expensive bit of a mobile network) has been used. They claim the aforementioned Boostbox is part of the mobile network. I did ask them where I can plug the ethernet and 240v AC it needs into the mobile network, especially as they have no cell site within 12km of my home. This just confused their simple Customer Care team.
My real point is that working this way means O2 core network doesn't need to do any accounting for Femtocell traffic. And so the situation is just as likely to have been caused by incompetence, or wilful inaction rather than a considered approach to the customer needs.
If it wasn't for the fact that I need to use the Femto I'd tell them where to stuff it.