The typical mattress will gain up to 20 pounds in the course of 10 years of use.
Also, I don't believe snakes have oil glands, and the fluid they secrete when shedding is between the old skin and the new one, not on top of the old skin, so baby shampoo should not affect it.
I'm not saying that soaping up a snake is good for it, I just wonder whether it's actually ever been tested by anyone to prove BAD for it. It may just be an assumption that it isn't a good idea. A snake's skin seems pretty impermeable to me...moreso than ours, wouldn't you say?