If you don't have an eligible replacement program installed - or Windows can't find it - it won't appear as a choice for default app.
AOL isn't recognized as a standalone e-mail app as skwire hinted earlier.
If you had something like Thunderbird or similar installed, you should be allowed to switch to it.
I don't think "none" is an option for the defaults. If you don't pick something, Microsoft will use its own products as the defaults.