Not just rentals this also applies to homes that you own and occupy.

Why does this seem to be a law enacted so 150 illegal aliens won't be living in one house or apartment? Probably because that's what its about.

It would also prevent two unrelated couples who are friends from sharing expenses by living in one house.
That seems too extreme to me.

So more then three adults in any given home naturally means that they are illegals, a rather myopic statement actually it just dumb. And with such a booming economy there is certainly no need to share expenses. And why assume every situation is going to cause a problem. Ames, the original city mentioned is college town, home to ISU. and wasn't enacted because of illegals, but to keep a bazillion students from living in a single rental. West Des moines just doesn't want anyone making less then 6 figures living in nicer homes then they deserve and messing up the suburban sprawl. How about if they are too noisy you fine them, or if they have to many cars on the street make them move them.