You're right, the 4G fee would not likely go away. You're also kinda right about the networks being built mostly by Clearwire. Sprint owns 54% of clearwire...so they are needing the money. They are trying to one-up Verizon and AT&T by having the 3rd biggest provider adopting something first...which means yes, they are taking quite a bit of a risk. Especially since wimax might eventually gets ditched and everyone holds hands and join LTE.
Here's a source for evidence:
http://online.wsj.com/article/SB1000...664233330.html