Of course they may have ulterior motives. Any action performed by a human might have ulterior motives. Anything anyone say might be a intentional lie to mislead.

But to assume everyone is lying until proved correct will not work. Reading news would be impossible, as would most other actives involving other people. The only working method then is to assume people are telling the truth until indication (not proof) is presented of the opposite. Blind trust would be only if we disregarded indication of falsehood when said indications exist.