You're confusing "trust" with "use," which appears to be the cause of your apparent bewilderment. I could be wrong, however.

Suppose you were handling snakes. Some snakes are not poisonous, and don't need to handled with the care you'd handle, say, a black mamba with. However, you are being advised to treat every snake you encounter as though it was the most poisonous snake known, and apply every care that you normally apply to snakes that you know are poisonous.

Will you handle the snakes? Yes, you're a snake handler, remember? But you handle all of them like they are deadly, even the ones you "know" to not be deadly.