Handcuff keys are actually pretty standard -- otherwise, the cop that cuffed you would be the only one who could open them.
But, yes, the keys would be 99% useless "in the yard" as prisoners are rarely cuffed inside the prison. (various maximum security scenarios aside.) And it has become increasingly common to use zip-ties instead of metal cuffs. (cuffs are expensive, use common keys, and some people can slip out of them.)