The bread & butter of the Internet is built around the idea that on the level of routers and switches ISPs do not impose arbitrary restrictions on traffic; an example would be how a Pakistan ISP once blocked the entire Youtube site for the entire world by routing all traffic to Youtube IPs over its network directly to the ‘shredder’.

Furthermore any site pays for its bandwidth already: you buy a certain data capacity from whatever company plays host to your service (website, torrent tracker or what have you), or you have your own servers and you buy bandwidth from your ISP (and you make sure that you can actually make use of your bandwidth).

In fact, this practice of buying bandwidth and then using it as you see fit is the business model of tier-3 ISPs: they simply buy bandwidth in bulk with service guarantees from a tier-2 or tier-1 ISP and re-sell chunks of it as ISP services to customers. These ISPs do not even have direct access to the hardware through which their traffic is routed.

About that blog; just reading on we get to some seriously funny stuff:
A variety of conceivable factors can come into play when a competing marketplace producer makes decisions of this sort. For instance, satisfying iPhone user demand for access to VoIP applications using 3G network capabilities could increase AT&T’s good will, cementing its relationship with existing users and attracting increased numbers of new users. On the other hand, giving iPhone user access to VoIP applications using 3G network capabilities could have the overall effect of subsidizing its VoIP competitors, ultimately resulting in loss of significant business to those competitors. There is the possibility that business lost to VoIP competitors will result in a loss of revenue necessary to cover AT&T's high, up-front sunk costs for both building and maintaining its 3G network, which like all networks, has finite capacity. And loss of revenue makes it less likely that AT&T can subsidize iPhones so that consumers can purchase them for less than $200. Multi-factored business decisions of this kind should not be made by regulatory fiat.
Just read that again:
There is the possibility that business lost to VoIP competitors will result in a loss of revenue necessary to cover AT&T's high, up-front sunk costs for both building and maintaining its 3G network
So if to pay for VoIP is to pay for your use of 3G, what it the usual monthly bill to pay for? To pay for your use of 3G, yet again?