It's because coccidiosis kills chicks really quickly. In a confined environment where the keeper checks the chicks on a regular basis the signs are usually seen early enough and treated. With free rangers especially if there are a lot of them with chicks you may not see the chicks for a couple of days and that could be too late.
The farm I worked on in Hertfordshire might have had a hundred chick, or more at any one time and you just couldn't check them all every day.
On my uncles farm it was just standard practice with all the chicks, battery and free rangers.
In Catalonia before I arrived they had had lots of chicks die very quickly mainly because they used an incubator and when the first feathers came through, they let them out onto natural ground. Resistance to coccidiosis is built up by low level exposure with chicks that don't get medicated feed but it needs to start as early as possible.