Sure, I understand the general principle of locking up your investment for longer and [perhaps] getting better conditions, but the notice periods on those accounts aren't government-mandated, though?

Sort of - to the FDIC, the difference between a savings account and a checking account is that a checking account is a "demand deposit" account that historically didn't pay interest and a savings account is a "time deposit" that does. Capital One is telling you that this savings account is a time deposit to legally categorize it as different from a demand deposit so the FDIC treats them as two separate categories and gives them both the $250,000 deposit insurance. It's federal law that requires them to delineate the two.