Your PDS webmaster has a signing key for your repo, because they for all intents and purposes are you. That's the trust structure of how PDS' are setup. If you don't trust someone else to modify your repo don't give them your private (sub)key.

The fact it's a subkey means that you are also able to rotate the key that the PDS has access to and modify your repo back to pre-defaced state if need be.