Is it really a fine line? Lyft explicitly say you can't use their API to compete with them. How can violating their terms of service be in any kind of grey are

Is it really a fine line? Lyft explicitly say you can't use their API to compete with them. How can violating their terms of service be in any kind of grey area? I'm not posing rhetorical questions. I'm asking you an honest question. Why would you think it's okay to use a service in a manner that the service explicitly prohibits?