I've noticed that third-party sellers generally get around this by having the same list price on their own site, but basically offering everyone a coupon for 15-30% off. Not just for signing up for e-mails, but spinning a wheel that pops up a discount, items that are on sale 95% of the time, etc.
So while this may very well be anticompetitive of Amazon, at the same time it's generally something savvy sellers and savvy consumers have been able to get around easily for a long time.