The equipment must be specifically authorized.

In the USA, the amateur radio service is, I believe, unique in that transmitting equipment does not need to be specifically authorized for use. This is called type acceptance. Licensed amateurs can modify and homebrew equipment at will, but are responsible for following rules regarding frequency, bandwidth, output power, interference, RF safety, and so on.

Every other radio service, as far as I know, requires that radiators (not just intentional transmitters) be type accepted.

Using an amateur radio to transmit on other services, even one that the person is allowed to use and following all the other rules, is therefore illegal.

I am not an attorney and this is not legal advice.