Even if you can place a business building on a public lot, you won't earn money from it. It's just not part of the game mechanics. The way to earn money from businesses is to purchase partnerships and then buy out the existing businesses in town... which is the Bistro, Grocery Store, Stadium, Science Facility, etc. You collect money from them every week when you have the deeds. However, there's a problem with collecting that money right now that hasn't been fixed yet. So, for the moment, we're stuck without business money.