This is an interesting issue. Currently in SF, defaults are never set when editing an existing page, whether or not that field (or even template) already exists. Normally I'd say that's the right behavior, since it prevents users from accidentally modifying a page's contents. However, in the case of a checkbox, which only has two possible values, maybe setting to "on" isn't any less neutral than setting to "off". I'm not sure, though - generally "off" feels like the more neutral/less "meaningful" setting. What do you think?

It didn't seem to apply even to new pages, if I remember correctly. Should investigate more.

I'm setting this to "wontfix" - on the assumption that this happens only when editing existing pages; and that thus this is expected behavior. Feel free to reopen it, though.