I think that's an effect of reusing the standard Print dialog box. The
ability to print selected text is globally available to any program, but the
program must support that ability. OneNote doesn't support that bility, so
it ensures that the option is greyed out when it calls the dialog box.
There's another thread from the last two days here where Kathy J suggests
creating a special section where you can temporarily place subsets of
information to be printed. That sounds like a good workaround for what you
want to do:
1) Select the text to be copied
2) Copy to the clipboard
3) Paste it into a blank page in your "Print Me" section
4) Print that page
5) Delete that page
