pr-auto-mode

pr-auto-mode is a variable defined in `printing.el'.
Its value is

t

Documentation:
Non-nil means major-mode specific printing is preferred over normal printing.

That is, if current major-mode is declared in `pr-mode-alist', the `*-buffer*'
and `*-region*' commands will behave like `*-mode*' commands; otherwise,
`*-buffer*' commands will print the current buffer and `*-region*' commands
will print the current region.

You can customize this variable.