If you use couscous in the usual way, to make the basis for a meal, then it is free.
But if you use couscous to replace something which would have a syn value, like the flour in a cake, then it is not free.
If you ate a bag of Doritos, you would have to syn them. So therefore, if you make fake "doritos" out of sheets of lasagne (which would be free if you were making a meal from them), then you have to syn them.
Basically, if you are using an ingredient to try to avoid another one that you would have to syn, then that's what they call "food abuse" (although I hate that term and wish someone would think up something else for it).