I cannot determine if the punishment provided was fair, but I must assume it to be within the legal safeguards of AR 27-10 and the UCMJ. This process is further protected when the Article 15 is reviewed by JAG for legal compliance.
With that stated, a Commander can consider daily performance and attitude to assist in determining appropriate punishment. Also keep in mind if you have done several things and are pending an Article 15 they must all be rolled into one Article 15 for punishment. So it is possible another offense or action was considered with regard to your punishment.