Yup that rule is laid down by amazon. How to fix it: many of the big mp sellers have their own website and if you order through them directly you can reduced shipping. I have to say though that I save so much buying on the mp that the shipping cost doesn't bother me a bit. 
One of the best is
www.importcds.com, which sells as importcds on amazon marketplace. Recently their prices on most of the premium import labels (bis, cpo, chandos, hanssler) are at about $12.50 per disc, and only $1.50 per disc shipping, with additional $1.50 handling charge on the first disc. Plus, they support paypal payment. Usually works out to a better deal than a new disc on Amazon marketplace.