On the one hand, definition of what is considered to be "able to ambulate" can be different between hospital and real life outside it. If patient "ambulates safely" within his room or even within unit, it doesn't authomatically mean that he would be able to do the same in his own house, or to walk those endless corridors. As a facility is responsible for a patient's well-being up to the last second of him/her being within it, it seems logical to ensure "safety" by, for example, not making patient ambulating and providing wheelchair even if he can perfectly walk on his own.
Secondly, I've noticed that the wheelchair seems to be included in what I name "entitlement package". There is a group of patients who really think that being pushed around in wheelchair, wiped after using toilet, given more of "good meds", calling for assistance for every trivial reason, etc. is somehow included into their hospital stay program. At least, that's how they describe "good care" if someone bothers enough to take time and speak with them. The very same people request home health care RN to come every morning to wipe them or provide them with wheelchairs during their outpatient visits although everybody just saw them jumping cheerfully out of their trucks. These "clients" are commonly "frequent fliers" as well, and they can be given what they demand just to keep things calm. After a while, some genius up there in Ivory Tower of Administration might start thinking that "customers" for some reason enjoy wheelchairs and "implement a policy of improvement" of this part of "our most wonderful customer service", thus mandating their use in all circumstances.
And, last but not least, something tells me that even in absence of such policy there still be that most impenetrable argument of "we ALWAYS do it this way here". Ingrained habit plus rationalization, one of the worst combinations to fight in human psyche
In this context, I find it interesting that patients who sign out AMA seem to be not authomatically "offered" wheelchair and expected to walk out of there on their own. I'd seen nurses and aides getting very grudging about pushing out a patient who signed AMA paperwork but was clearly not fully ambulatory and needed help to get into private car with family waiting in it on "drop in/off" spot. Also, several times when I was in ER I was told that I had to stay in my room and that walking 50 feet to the ice machine in the corridor was "unsafe" (I was quite obviously able to walk, A, Ox4 and not on any monitors or oxygen at that time). Once I signed my AMA half an hour later, it suddenly became perfectly OK for me to get water myself and do not bother my poor CENA who was clearly at the very end of her wits and will that night.