When you open a CD, you enter into a contract involving a fixed amount of money (principal) for a predetermined period of time (the term) and an agreed-upon interest rate and yield. The bank is complying with the terms of the contract. It is not obligated to change those terms when interest rates change, unless the terms of the contract require a change in interest rate.
Review your account agreement for policies specific to your bank and your account.
Refer to 12 CFR 1030 Truth in Savings Act (Regulation DD).