Control doesn't equal ownership. Without 100% ownership there will be network management problems. Also Access to Spectrum isn't free. S pays $550M a year to CLWR for access even though S owns half the spectrum. It would $1.1 BILLION a year if they owned none of CLWR. Demand is going up and the amount Sb and S have to pay is going to go up. Why pee money away on that. After 10 years of this, they could have bought the spectrum and saved Billions.
What you're saying isn't true. In order to avoid a conflict of interest and for Clearwire to perform its fiduciary duty to shareholders, Sprint has the pay the same for the access as they would if they owned 0% of Clearwire. This increases the value of Clearwire (aka share price) which benefits shareholders. They are theoretically "rewarded" through share price, but cannot be given any non-competitive discount.