The same argument would make you not want to drink water from a public fountain, or eating at a restaurant as opposed to cooking your own food. It's a matter of

The same argument would make you not want to drink water from a public fountain, or eating at a restaurant as opposed to cooking your own food. It's a matter of implicit risk analysis. You need to charge your phone, you see a USB port. What are the odds that it's going to be wired to something other than 5V? If that were the case, what's the worst that can happen? It works out for the vast majority of people.