depends on the city you were in. Detroit didn't allow food trucks downtown until sometime in the last 12 months; I think the change was actually not long before the baseball season started. If the same ordinance applied to other areas of the city, it probably wasn't being enforced. Sandwich trucks used to work downtown Pontiac back when more people actually worked down there. No idea how Garden City did it. The few articles I've read on the subject concentrated on the Metro Detroit cities that are entertainment destinations - downtown Detroit, Royal Oak, Birmingham, Novi, Wyandotte, Plymouth and Ferndale. For whatever reason, Pontiac and Mt. Clemens weren't mentioned, even though they have lots of nightlife.
Kinda off-topic, but last I read ( maybe 2 years ago) open grills/fires of any kind were also banned in downtown Detroit. That seemed to have something to do with the EPA and overall air quality issues in downtown.