Tiggs: It is true, Corn does contain protein, and they basically use one specific part of the corn for indivudul ingreidents.
Now... for humans, yes corn is a good food to have, but even some people specially infants, can have a tough time digesting corn, which leads me into talking about cats...
Cats... are basically unable to digest corn, in any form be it corn gluten meal, ground corn...etc.
So, corn is a protein source... just not for cats, most cat owners don't know this, so some companies like SD, Iams, Nutro etc... could care less, it takes up a lot of room in the food, a lot of that food is just corn, which fills up a cats belly (a filler food if you will) tricking them into thinking they are full, but in the long run, they just poop it out... can't use it, no nutrients come of it etc.
Now you might be saying "well why is this allowed in pet foods, I don't think it's that the companies (don't care.)"
When it comes to the AFFCO as long as the ingredient doesn't have overwhelming proof of posionus or other ill effects it can be put in the food, to any quanity you want just about. Plus it's cheap, so a lot of companies decide to use it, except for those select few, I am very greatful to, who take it upon themselves (basically smaller brands) to make a better food, because they care about animals, they constantly study whats best in nutrition, not whats cheapist in nutrition.
This goes a lot deaper, but I will stop here before I completely confuse you. lol
I hope this helped!