No signs in my Walmart.
But they have a right to dictate what you can and can't do on their premises and inside store, and if you refuse, they can certainly call the police.
for a while people would sit outside and sell pit bull puppies. That is no longer allowed, for instance. As a matter of fact, a lot of people got kinda tired of seeing the front of Walmart turned into an "unwanted animal adoption" station, some days there would be 5 people out there w/boxes of pups and kittens.
So our local walmart did have to do something about this. It is a private premises....people need to respect that, it's not a free for all in the parking lot......you have to ask permission
Keep in mind, the role of Walmart in a very small town is quite diff than in metro areas. This is where the Salvation Army, the schools (car washes), the free innoculations for rabies for your pets, and many other charity and fund raising groups have their events. And in a small town it is Walmart who gives generously when there is a tornado, they still let RVs and people on long trips "nap" in the parking lots, etc.
I had a nice benefit there for the wildlife rescue.... they allowed us to bring in baby deer, straw bales, fenced enclosures, etc. and set up a petting zoo of sorts, as well as some very rare periguine falcons and birds of prey........all the things we rescue ......and need $$ to feed.
Walmart allowed us to do this for an entire weekend, at no charge, and let us set up right in front by the doors. Very nice.
So Walmart performs a vital function in many small towns across the US
When I was writing a grant for a community garden they were going to kick in a lot of free garden center stuff for the project......