Unfortunately in America insurance scam is somewhat common. Many insurance policies from auto, home, health, and collectables have "waiting" periods before they go into effect. Health insurance is slightly different as most employers will insure right away from the first pay check but sometimes they too have probation periods. I have seen cases where new hires are given health insurance after 90 days, especially in minimum wage, high turn over situations where someone comes in, gets hired, gets the medical treatment, then quits.
Even in one case I saw a person not having life insurance and they knew they had a terminal condition. So they hired into a position, bought the premium life insurance, etc., then the person passed away a few months later and all the benefits paid out to the family.
So the open enrollment is more to protect the company. Insurance is built upon people who do not use it, if you think about it. It's like auto insurance or home insurance. You're paying into it based on your area, how many claims were made, how many people are insured drivers, and general disaster areas. So the more people who do not use the insurance, the cheaper the premium is to you. If a lot of people are using insurance, or making claims, then the rates are higher. Health insurance costs an employer a lot of money as they share some of the burden of the payment. With that said, if they can "lock you in" to the rate for a year, and hope you don't use it as often.