Where this falls down is in situations where the broadband provider and the femtocell provider are different companies. So perhaps Orange femtos linked to BT ADSL, or AT&T femtos on Comcast cable.
There are two issues here:
- Extra traffic load for the ISP
- Competitive or net-neutrality arguments about value transiting "my pipes".
(a third issue - how the mobile operator gets QoS on 3rd-party broadband, is also involved).Now if the femto traffic is just "best efforts" mobile voice being offloaded, I wouldn't expect most of the broadband providers to worry about the incremental traffic load. But if it's 3GB a month of "dongle" traffic from a PC, going to a mobile broadband provider, that's a different issue. In that case, I'd expect to see traffic-shaping being used, perhaps throttling femto traffic to 100kbit/s or whatever level is applied to other bandwidth-hogging services like BitTorrent.
The competitive argument is a trickier one, and will vary according to markets. In countries where all the main operators have both fixed and mobile arms, any attempt by one to unilaterally block (or charge for) another's femto traffic could lead to a nuclear-style "mutually assured destruction" scenario.
But in other markets where there are dominant ADSL or cable providers that don't have mobile arms, I'd expect some to become pretty aggressive about their femto traffic-management policies. It's not as if it's hard to spot femto backhaul traffic on the network, as it goes to a fixed IP address of the gateway & will have a recognisable profile.
We may see some broadband ISPs attempt to negotiate with mobile operators to guarantee QoS on their lines, for a a price. Some may even return some proportion of cash to the end user, so they don't end up "paying the operator's backhaul costs".