This is undoubtedly a requirement enforced by the system administrators. The only way to get rid of it will be to remove the Exchange account from the phone. If this is a problem, talk to your IT department. Please don't take this personally, as it's not meant as an affront to you, but as a sysadmin, I can tell you right now that if you complain, they'll tell you to go pound salt. They require the passcode for security. They'll tell you that if you don't like it, then you don't get your e-mail on your phone.