I don't know about Twitter in particular, but it's fairly common for users to log onto accounts for various services on borrowed devices and then log out when t

I don't know about Twitter in particular, but it's fairly common for users to log onto accounts for various services on borrowed devices and then log out when they're done. So the device is only partially trusted, or only trusted for a limited amount of time. If you want to say you'd never enter any credentials on any device that wasn't fully trusted, well 1. That just doesn't match how many people use accounts and devices, if you want a widely used service, you must account for it even if you'd never do it, and 2. It will still cause problems if you ever sell a device, or have to return a corporate/managed device after leaving a job, or return a device for repair, etc.