This actually has very little to do with ISP's and everything to do with NSP's.
A NSP (network service provider) are the companies that own the backbone ie. the actual cross country cables that allow the internet to run.
The ISP's buy the privaledge to tap into this cable in certain areas to provide internet to communities and businesses.
Say the NSP's decide to tier the internet to ISP's they in turn will be forced to do the same.
Some people say this could never happen because other companies will just pop into place and not follow suit and put all the big guys out of business.
But the reason this isnt true is because the gov wont allow the construction of New NSP pipelines.
So in essesence if a rouge ISP wanted to just not join in on the tier program, the NSP would just shut them down.
Again THIS HAS SO LITTLE TO DO WITH ISP'S AnD everything to do with NSP's
NSP = Companies that own the PIPELINES
ISP = Companies that use the pipelines to provide internet