Interesting so as I understand it cask and keg are names for a container but with beer one expects the container called a cask to contain live beer and one called a keg to contain beer which has been processed to keep better with less care required. But since they are just the names assigned to a container which could just as easy contain gun power or nails the calling of a beer cask is no guarantee of type of beer.
Since traditional ale contains no hops then "cask ale" should in theory be as would have been made hundreds of years ago and you would have the strong first ale and weaker ale the latter used as a purified water kept pure due to alcohol content. However it is unlikely and beer bought in the pub is of this type the whole idea of drinking light ale instead of water is just a historical thing and today we expect a completely different brew.
The whole question is one of marketing commercial beers and transporting them and since in the main we don't transport home brew it is really anything to do with home brew however to discus home brew we do need labels to describe taste nice and horrid is not really enough. As a result we compare to commercial brews so we have.
Ale, Bitter, Lager, Mild, Draught, are names we associate with a taste and although Lager refers to a method buying a can of concentrate and processing it without using the lager process but with a taste similar to beer which does use that process we would still call it Lager as we are describing the taste not the process.
I expect bitters added to my beer and to drink a "cask ale" with no hops added I would likely not appreciate the taste in the same way as if you ask for coffee and are given tea you may like them both but is given the wrong one you don't like it as your brain is expecting something else.
Tradition is not easy as with something like IPA was designed to be transported and keep in warmer climates and the paler it becomes the less alcohol it has read Wikipedia it states most ale contains hops ask an historian and the tell hops only started to be used in the 13th Century and traditional ale has no hops.
So in real terms we are not talking about beer making but how to use the English language to convey to others a taste found in a beer so if it tastes like a commercial keg beer call it keg and if it tastes like a commercial cask beer call it cask.
To my mind if there is live yeast in the beer it's a real ale if it's been sterilized it's not. It does not matter if heat or chemicals are used. I would expect anything with cask label to be live and with keg label to be sterilized however if a pub was to swap labels I don't think they could be taken to court for it as just a matter of trust and way of describing the produces taste.
Other descriptions may be different within the food trade to butter bread one must use butter as margarine can kill some people so asking if you want some spread on it may be a better phrase but as far as beer goes I am unaware of any requirement to say things like may contain traces of dandelion, burdock root, marigold, horehound, ground ivy, or heather however I could also be nuts!
It would be interesting to hear if there are any legal labels required may be they have to have a warning "May contain alcohol"!