A Ponzi or MLM use a single source of supply to create a profit center. You can buy bitcoins on the open market. That would be like buying your competitors products to sell and would kill your pyramid.
MLM people like MLM. They would get all their people in just to make a profit off it.
Freedom isn't cheating. However, one of the things that I am looking for is ideas for making it difficult to do the thing that might be considered cheating by some... to guide folks into the direction of using it as it is. There might come about alt-Ponzicoin clients. But, if we don't do it with Bitcoin, somebody just might apply this idea to Litecoin, or one of the others, that has its own blockchain.
The point is, nobody could control a Bitcoin MLM that was built right into the client. If it caught on in the MLM croud, Bitcoin would explode.