A metal roof is more durable and damage resistant than shingles, and that translates into less risk for the insurance company to insure. Just as a brick or stone home is cheaper to insure than a wood home, a metal roof is cheaper than shingles. It is not so much that you are being rewarded for the metal roof, it is just that having one reduces the risk to insure the home, and that savings is passed back to the customer.
A lot of home factors are taken into consideration when setting your homeowners insurance rates. The size and location of the home are important factors, as are the age and materials the home was built from. You can also get cheaper rates if you have a fenced yard, burglar bars or deadbolt locks. All of these things make the home safer and less risky to insure, and together they can add up to a noticeable savings.
You provide some additional factors for the insurance company to calculate, as well. Your credit score is going to be important to determine your financial stability, for instance, and people who have acquired a Bachelor's degree or better in college will earn lower rates than someone with a high school diploma. For the insurance company, a lot of the information that you may take for granted on a daily basis can be translated into the risk they face when writing you an insurance policy.
The point is that even though a metal roof is less expensive than shingles, it is only one of many factors that your insurance company will consider. The more you do to reduce the risk posed by yourself and your home, the lower your rates will be. Talk to your insurance agent and ask about other discounts that may be available to you, such as multiple policy discounts or a homeowners discount for having a professional security system installed.