It also has the (not undesireable) side effect of rewarding customers who pre-order directly from Mongoose, since that is their biggest profit margin.
If you want a book fast, order from Mongoose. If you want it cheap, order from Amazon or somesuch and wait (or wait for the PDF).
Me, I support my local FLGS, so I get the best of both worlds: I pay full price AND get to wait! Typically it takes 1-2 weeks after the official release for me to get a book.