Travis legally has control of the company just like an owner would. Nobody can "force" him to leave on run his company as certain way. Benchmark is just whining because they can't get their way.
Benchmark agreed to let him have permanent control when they invested, if they didn't want that they shouldn't have signed the contract.
Invisibly stapled to every contract is the entire corpus of relevant law. Delaware, for instance, protects minority investors from certain categories of abuse. If the claims in this letter are true regarding Kalanick telling Uber's Board that he was going to sign certain voting rights agreements, and then he did not do that, it could be grounds for finding violations per those laws.
Disclaimer: I am not a lawyer. This is not legal nor investment advice.