check out mana.org for some state-by-state info to help understand that laws for CPMs vary HUGELY by state. MANA is the midwives alliance of north america -- the group doesn't focus on breaking up midwives into all of their respective certs (CNM, CPM, CM, DEM, LM, lay midwife, etc.), but instead trys to encourage good standards of practice across the realm of midwifery (to wildly reduce their entire mission statement... sorry MANA).
here's the state by state breakdown of laws for direct-entry midwives/CPMs/LMs (so many terms that effectively point to the same person -- a non-CNM midwife):
Direct-Entry Midwifery State-by-State Legal Status
in most localities, i don't think it's as easy as 'not needing backup' as you mentioned, whether legally or culturally in your area. there's usually more to it than that. for instance, some states require licensing and in order to get your license you need to have backup -- it doesn't have anything to do with the hospitals in your area, but the licensing board of the state.
i applaud whichever route you take -- i started training as a CPM but am now waiting to hear back from CNM programs... we need more midwives regardless of what letters are on your license, i say! on either side of the coin (CPM vs CNM) there are midwives i would want at my baby's birth, and there are midwives i wouldn't even let in the building... they're just letters, in the end. go the route that makes the most sense for YOU and be the best midwife you can be.
this is a topic i feel strongly about and love to discuss so let me know if you have any more questions.