I think its because its less taxing on your body. The oil pulling can work on toxins without having to deal with the food content. It doesnt hurt to do it if you have eaten but it works better and more easy if you have not eaten.
I have been oil pulling for nearly 14 months and I am not sure I have ever done it on an empty stomach, but have still had tremendous results. I wonder what will happen when I can actually do it first thing in the morning????!!
Because, it appears that OP stimulates the gall bladder to contract. If you have food in your tummy, you may get a stomach ache. But maybe not. You do Liver Flushes on an empty stomach, so OP is like a mini Liver Flush really.
Thats the thought anyway. I know that sometimes if I have an empty stomach, I can feel my gallbladder contracting while I do OP. Wild.