TL;DR: If a user can add channels to their devices via the website, they should be able to sign into those services as well.
This option only appears when linking a new device to an account. This needs to be integrated more deeply into Roku and its OS. As someone who uses a password manager, signing into channels can get rather tedious. Unless a channel developer provides a pairing mode, using the remote directly is almost entirely out of the question.
Users should not have to trade security for usability. If people are to become more security-minded, Roku and other platforms should do everything possible to prevent people from using bad passwords, much less repeat them across multiple sites.