Why Does Revoking a Token Allowance Require a Blockchain Transaction and Incur a Gas Fee?

Revoking a token allowance requires a blockchain transaction because the allowance itself is stored as state on the blockchain. Changing any state on a decentralized ledger, whether it's transferring tokens or changing a permission, requires a new transaction.

This transaction must be validated by miners or validators and included in a block. The gas fee compensates the network participants for the computational resources used to process this change and secure the network.