It has been requested before.
The reason for the current approach is that there are situations where the parameters entered by an end-user are illegal to use (e.g. certain frequencies). As a network-operator (assuming this is not the same user), you want to make sure that devices are compliant with local regulations. In my opinion, this should be the responsibility of the network admin (able to make modifications to the chirpstack-network-server.toml file), not the end-user.