The main reason for the higher CYA level for SWCG pools that are outdoors and exposed to sunlight is that the higher CYA level, even with a proportionally higher FC level for the same active chlorine level, loses less absolute FC to sunlight (see
this post for a chart that gives example loss rates at different CYA levels). That lets one turn down the % ontime for the SWCG and this not only extends cell life but results in a slower pH rise due to less aeration from hydrogen gas bubbles and less possible undissolved chlorine gas.
The SWCG manufacturers recommend the higher CYA primarily because they recommend undersized SWCGs that wouldn't be able to keep up during the day with the chlorine lost from sunlight without the higher CYA level. They don't have their FC levels high enough at 1-3 ppm so in a pool rich in algae nutrients such systems will at first appear to have a mysterious chlorine demand and be unable to maintain an FC level (unless oversized) because nascent algae growth is not always visible.
So you might ask why we don't recommend 80 ppm CYA for non-SWCG pools since that should result in less absolute FC loss per day so be less expensive. The main reason is risk. Because of the SWCG dosing frequently, there is less risk for having the FC/CYA level get too low while with manually dosed pools the risk is higher. If algae were to get started, then doing a SLAM with 80 ppm would be a lot of chlorine -- 32 ppm FC -- compared to having 30-50 ppm CYA where a SLAM only needs 12-20 ppm FC. Nevertheless, in areas with a lot of intense sun, some manually dosed pools go higher than 50 ppm, even up to 80 ppm. It's up to the pool owner so long as they understand the risk.
There's a related question that has to do with why the FC/CYA ratios are lower at 5% with SWCG pools compared to 7.5% for non-SWCG pools. Experience has shown that the SWCG pools don't need as high a minimum to prevent green and black algae growth, but we don't know why for sure. Some of it might be the higher salt level. Algae can still grow in that level, but perhaps its rate of growth is a little slower. Another reason is the more constant dosing from SWCG pools though non-SWCG pools that use automated dosing systems (peristaltic pumps or The Liquidator) seem to need the non-SWCG minimum FC levels so that's probably not the reason. Another possibility is the superchlorination in the SWCG cell since it is high FC at low pH near the chlorine generation plate. This would certainly kill off a portion of the free-floating algae getting circulated through the cell but wouldn't handle algae growing on surfaces.
