view-read-only

view-read-only is a variable defined in `files.el'.
Its value is

nil

Documentation:
Non-nil means buffers visiting files read-only do so in view mode.
In fact, this means that all read-only buffers normally have
View mode enabled, including buffers that are read-only because
you visit a file you cannot alter, and buffers you make read-only
using C-x C-q.

You can customize this variable.