Event coupon codes bypass membership checks/settings
When someone registers for an event using a coupon code, it bypasses all membership checks and conditions. Thus, when a member mistypes their e-mail address when signing up for an event using a coupon code, a non-member contact is created for the registration, and the member's account is not tied to the resulting registration. This messes up our contact database, all automatic e-mailing (usually the e-mails will bounce so the registered member doesn't get any communications), the organizer's records, and the member's membership record.
We have very good reasons for wanting to limit coupon codes only to members. For example, this is a way to give the member event organizers a "deal" for all their hard work when they sign up, it allows us to allow discounts for speakers or instructors who are members, etc.