Well since I dont have a me.com email to play with, i did a little searching and it seems it is due to the phone treating it as a IMAP "push". Below are the full details
the phone is not treating your account as POP, it's treating it as IMAP. This makes your situation completely normal, however; on WP7, there's no push support for IMAP accounts.
AFAICT, WP7 does not support IMAP IDLE, which is the "push" version of IMAP. (Neither does the iPhone, FWIW.) IIRC, on the iPhone, MobileMe push is accomplished via Apple's Notifications system, which triggers a "pull" any time a new MobileMe mail comes in. (Since MobileMe mail is hosted on Apple's servers, Apple, of course, "knows" when you have new mail and can trigger the iPhone to fetch it.)
On WP7, push is implemented via EAS (Exchange Activesync) which is supported by Live/Hotmail, Gmail, and, of course, Exchange. MobileMe doesn't support EAS, ergo there's no MobileMe push support in WP7. IMAP IDLE, from the various third-party solutions I've tried on phones over the years, is murder on a phone's battery, so I suspect that's why most if not all phones eschew it for other push methods.