One counter example to your claim is Houston. While it exhibits the same urban sprawl, there is no legal master plan that divides the city to different uses.
Houstonian here. What you say is correct --- but many, many neighborhoods in Houston have legally-enforceable deed restrictions [1]: When you buy property in those neighborhoods, you are deemed to have agreed to a (typically, very-detailed) set of restrictions about what you can or cannot do with, or on, your property. Local homeowner associations can be pretty vigilant in seeking out and going after violators.
Moreover, some neighborhoods are in fact separate, incorporated cities that do have zoning laws. (I live in one such.)
[1] http://www.houstontx.gov/planning/Neighborhood/deed_restr.ht...