Real ales are not supposed to be pushed with CO2 at all and you are right cask conditioned ales are still fermenting in the cask. A cellar person will let the secondary fermentation occur and control carbonation levels by using something called spiles. They are hammered into what is called a bung hole (yes, that is really what it is called) and allow gas to escape. Hardwood spiles will hold pressure and the soft wood allows it to escape. After perfecting the carbonation levels in the beer and allowing it to mature the beer is pulled through the system with the beer engine. The air that is allowed into the keg can cause the beer to spoil if not consumed fast enough, that is why some will push it with CO2. Bars just need to manage their casks and be certain it sells in time. The beers will change flavors too as a result of the air, it is interesting to sample the same beer at different times over a day or two. There is an organization named CAMRA (Campaign for Real Ale) that has a ton of stuff on their website if anyone is intersted. The website is
Campaign for Real Ale