Why is it impossible to set a speed below 30 km/h on every Tempomat I've ever seen? Is it for legal reasons or technical difficulties with low speeds?
It would be helpful in a stop and go situation.
Generally a 30 zone implies you are in town, where you are subject to children, pets, footballs, stopping traffic etc.
Cruise control is for long distances at a constant speed where you are not subject to rapidly changing risks - not for stop-go situations. In town you should be concentrating on what is going on and adjusting your speed constantly based on what is around you.
Specifically to your final sentence - Cruise control is the opposite of what you want in a stop go situation!