Anything that allows working with multiple documents should remain open even with no windows (or unless auto-killed).
Mac is document-based instead of application-based so closing document doesn't quit the app. it only does if multiple documents doesn't make sense to the application like System Preferences.
This is the HIG quote from above:
> "In general, quit when users close the last open window in your app. [...] If users close the last remaining document window in a document-based app and switch to another app, it’s appropriate to quit the app" (emphasis added).
In TextEdit, close the last document and then _switch to another app_. You're missing the last step.
