USDC is supposedly backed by USD. If Cirle/Centre doesn't have access to enough USD, they cannot maintain the peg of USDC.
If I move USDC from one wallet to another, and someone else moves bitcoin from one wallet to another in exchange for that USDC, that transaction happens entirely on the blockchain and at no point involves a bank or a peg.
The only reason to disable that function is to force people to hold USDC.