This makes a lot of sense, because RSS is quite e-mail like anyway in so many ways: the feeds look like an inbox, where items are marked read and such.
Also, a lot of sites use email where they could (should?) be providing RSS, for updates and news stuff. So to see those in the same place as RSS feeds, either you use a common client for both, or convert one into the other.
Feedbin seems to permit receiving email newsletters as a feed, so that's one of the few making email into RSS, whereas the others all turn RSS into email.
Like some others have noted, using email as a storage mechanism reduces part of the problem (tracking which items are read/unread) to one that's already solved (by IMAP). Additionally, using isync lets me have local copies of everything; this used to be really important when I was a "poor" grad student, because I could do cool stuff like download a bunch of comics ahead of time on my laptop, then read webcomics/mailing-lists on the 2-hour bus ride. I still like having local copies of things on principle, although nowadays everybody is always-connected so it's not as useful.