I have a login on my front end, if a member logs in and then clicks on a link to "edit_this" which is the shortcut to the back end to edit the entry, why do they have to login again on the back end?
Most likely because they are logged in with
www.domain.com but visiting the cp with
domain.com or visa versa.
The solution is to adjust the Cookie settings for the EE install. Specifically, set a Cookie Domain so the member's logged in session will be used across both URLs.