Depends on where you live, I suppose. Large cities I've lived in have all allowed commercial spaces at ground level, but there are a ton of regulations as to what you can do in them.

Doing nothing but knocking down a wall and putting up a sign is a sure way to get shut down by the city, along with some likely hefty fines.

Edit: another barrier is building insurance; if a building is all residential, and someone wants to convert ground level space to commercial use, the insurance company has to re-assess the risk and premiums they charge due to the increase in people coming through, if nothing else.