The point being debated is not about saying homeless can do as they please. It's very specifically arguing you need to give the homeless some place where they can camp out or other wise live.
If you had a city that designated all areas under bridges as legal for homeless occupation, then yes those cities would be allowed to remove the homeless from other areas.
The entire point of this case boils down to "do homeless people have a right to exist" not "can homeless people sleep on my kids playground if they want". The point being argued that cities have some obligation to take care of their homeless populations and if they refuse to, then they are essentially giving up control of their public spaces. Want a nice downtown? Great, give the homeless sleeping on the streets there some place else to go and yes you can force them to leave.