I'm not sure why the language was included to specifically target Detroit since that is the only city in the state with a population of over 900,000 people. One of the co-sponsors, Rep. Leland, represents a part of a small Detroit. I am not sure if he has any urban farming in his district, but I hope that he did this out of requests from the growers in his district and across the city of Detroit and not as part of any paternalistic philosophy.
Fortunately (or unfortunately, depends on how you look at it) this won't affect any other urban farming efforts across the state including Flint where I live. We have similar land use issues and there are several urban gardens/farms on lots in residential neighborhoods.