In a corporate environment running Exchange and Outlook clients connecting to it why do server administrators not allow IMAP to be turned on for those that would like to connect to the server using IMAP?

I'm assuming the simple answer is port attacks and please excuse my naivety but does Exchange not suffer the same problems or is it because its behind the firewall and within the corporate network?

**UPDATE:** Thanks for the replies. Can anyone confirm that if IMAP was turned on there would be a susceptibility to outside attacks?