I am not a lawyer, but as I understand it:
A member of the public is invited into Walmart or the CTA for specific purposes (to shop or be transported). A conditional part of that invitation is acceptance of their rules of behavior while on their property; that is, if you enter their property you agree to follow their rules. Note that restaurants have such rules, ranging from requiring simply a shirt and shoes up to requiring formal attire. Stores don't allow soliciting. These rules are nothing new.
If someone enters those properties and violates the rules, they lost their invitation to be there. They may be asked to leave, and if they fail, they are cited for trespass.
It does make sense. If you invite someone over your house for lunch, you assume they're not gonna rifle through your bedroom drawers. If you discover them doing that, you'll ask them to leave -- you're withdrawing your invitation -- and that person is now trespassing on your property.
One controversial issue in transit carriers is restrictions on photography. Some carriers have _always_ banned photography, mostly out of a very justified fear the photographer may trip and hurt himself or others and the carrier will get sued (happened a lot). People who wanted to take pictures had to get a permit and sign a liability release.
More recently some carriers, as well as some toll road authorities, have banned photography as a 9/11 security measure. This is controversial because people question if it really enhances security. NJ Transit had such a ban and proposed to make it stricter, but after a public outcry they removed the ban altogether. (In all cases, photographers are still restricted not to do anything to impede trains, other people, etc., quite properly so.)
I am not aware of any ban on the CTA, at least no one bothered me when I took pictures.
NYC once had a ban for liability protection but lifted it. The Port Authority (PATH trains) has an extremely strict ban on photography. I guess being hit twice by terrorists made them nervous.
A number of years ago Bell had an open house at a crossbar office (normally these places are very secure with no visitors). I went down and asked if I could take pictures. For some reason that set the hosts into a panic and they didn't know what to do. Finally they said yes but without flash. The flourescent lighting was just barely enough so I got a few pictures, though I don't know what I was taking (except the AMA recorders and a DAB operator who posed for me.)
I was given a tour of a modern ESS office and permitted to take pictures with flash (no flash in the basement where the batteries were). But there was nothing pictorial; just a bunch of boring blue boxes.
[TELECOM Digest Editor's Note: CTA has (or did) have an almost constant 'war' with 'street people' loitering in their stations during overnight hours in cold weather. They required people of course to prepay their fare, which met the 'intention to ride' requirement and often times the person would secure a 'transfer' to ride a later vehicle without having to pay any additional fare, then go down in the subway and (as they had intended to do all along) fall asleep on one of the benches for the rest of the night. If a 'rider' fell asleep sitting or laying on a bench and was awakened by a police officer or transit security person, that person always managed to have a very innocent excuse: oops, I fell asleep and missed my train, now I will have to take the next train (which run through the subway at 15 minute intervals minimum during the night, and depending on where you claimed was your destination, 'your' next train might be another 30 minutes in arriving! Here is my transfer to 'prove' where I was going to. Am I not allowed to sit here and close my eyes resting while I wait? I will try not to fall asleep again. (oh, sure you will.)CTA retaliated by remodeling the benches to put arm rest dividers on each bench several inches apart from one another making it physically impossible to lay down on a bench and use it as a bed. To add insult to injury where the street people were concerned, CTA then painted a sign on each bench saying 'this bench is intended to provide seating for five people' (and by implication 'not just one person laying down on it'. They also put up signs saying 'do not go to sleep, do not close your eyes. Stay awake and alert on our property.' (Citing some recently passed city ordinance requiring same.
With Walmart (there are _no_ Walmart stores in Chicago itself; only in a couple of suburbs; that is because the Chicago politicians have various disputes with Walmart executives over things like Walmart's pay scale, non-union practices, etc), in one of the net newsgroups (I think it is