I see two possible explainations, but I'm wondering if there is another.
1) Timing: They won't gain control until or at such time as they designate 7 directors.
2) Level: They won't gain control over all actions such as those that require 10 of 13 board votes or a 75% shareholder apporval.
Both are weak arguments for saying they won't "obtain the right". If an investor really believed the McCaw deal and Softbank Sprint merger was going to be negative for Clearwire's minority investors, now would be the time to file the lawsuit. But I don't think that is the consensus or goal of the letters.