Wi-Fi offloading is getting traction recently, with carriers deploying large networks of "carrier Wi-Fi" hotspots - usually in order to reduce congestion and churn, as well as preserving QoE (see - KDDI, MetroPCS).
In most cases, when a mobile subscriber is offloaded to a carrier Wi-Fi, the service is free; data usage does not count towards the monthly quota (for example - AT&T: "Using Wi-Fi is convenient and helps you save on your mobile data usage").
Why? For technical reasons? Commercial?
".. The problem is, that when CSPs offload their mobile data traffic before the GGSN they cannot meter that traffic; one of the functions of the GGSN they are also bypassing. When I pointed this out to a number of operators at MWC they retorted they didn’t wish to charge for offload so that didn’t matter. Firstly, this is a mindset issue, and it is also an artefact of the industry in many developed markets. In contrast, CSPs I spoke to from developing markets were far more likely to understand the problem and say “why wouldn’t we charge?” The point is that in future you may wish to, and if you don’t build in this capability (for example, by sending the traffic through a gateway) then you cannot easily change your mind in future. Furthermore, you build up an expectation that traffic offloaded to WiFi should be “free”. As with all-you-can-eat data plans this can be a hard expectation to change" (see also "Smarter PCC Solutions Open the Door for Wi-Fi Charging"- here).
"It is critical that CSPs regard offloading as an integral part of the service they provide, and not just as a knee-jerk reaction to congestion".
See "WiFi offload – beware the hidden commercial dangers"- here.