Some birds can have it due to obesity rather than fluid, I don't worry much about birds not showing any signs of shortness of breath, rattling, listless behavior or other signs of illness.
Another cause for ascites is cancer, some hens get it when they have reproductive tumors, they usually don't show respiratory issues till it is extreme and the fluid pressure is compressing the internal organs and interfering in circulation.
The reason it is a concern with the meat birds is because it is developing in a bird who is only weeks old, usually a sign of CHF because their circulatory system can't keep up with the workload caused by their abnormally fast growth rate. When CHF becomes acute in these meaties it is not going to end well.