Speaking broadly, that's not true. Monopolies are not intrinsically illegal. Antitrust law is specifically about business practices, and the notion that monopolies have to be more restricted because they can use their domination of one market to affect related markets. What got Microsoft into trouble wasn't having 90% of the operating system market, it was the allegation that they were using that monopoly to take over the nascent browser market, force OEMs to accept terms that prevented alternative OSes from competing on level ground, and so on.

I suspect Amazon is on their way to running afoul of such laws, or would be if the current business climate wasn't comparatively hostile to the notion of this kind of regulation. I don't think, however, that Zocalo is going to be a piece that attracts much attention; it's going to be the way Amazon seems to increasingly deal with their suppliers in Walmart-esque fashion. (Walmart was notorious for going beyond merely asking for "large customer" discounts and heading into "we will tell you what wholesale price we're going to pay, and you will either make it work or you'll lose the 60-70% of your business we represent.")