Cats usually do quite a bit of licking, cleaning and grooming of themselves. Of course this means that they ingest anything on their fur and skin. All the more reason not to use products that contain chemicals and pesticides when controlling fleas.
Even for super sensitive cats neem oil shampoo is very safe. You do need to be aware that every neem shampoo that is safe to use on dogs is not always safe to use on our cats. It is not the neem oil in the shampoo that is the issue, but instead it is the other essentials oils, especially tea tree oil. Cats are sensitive to the smell of essential oils especially oils that are usually used to hide the scent of neem oil.
Make sure when you buy neem oil products for your cat that they do not contain tea tree oil. Better yet, buy 100% pure organic neem tree oil or mix it with your regular shampoo you use for your cat. The shampoo is rinsed off of the cat so ingesting it is not a concern. There is no need to worry about neem oil traces since they do not cause any harm. The fleas are discouraged simply by the smell it leaves behind.
Neem leaf is the preferred part of the tree used for treating many skin disorders and ailments in Ayurvedic medicine. Even if it is ingested in large amounts, neem leaf is totally safe. People look to neem oil to use as a natural insect repellent.