Of course, Amazon is not techincally the only store. But they are large enough to have substantial market power (and, indeed, the current dispute proves it: it would make no sense for Amazon to embargo a publisher if Amazon had no reason to believe this would do any harm to that publisher). You also need to take into consideration the prevailing trend in the market (power accruing to Amazon) and what this says this about how Amazon will behave in the future (use of power to benefit itself).
As much as I dislike publishers: replacing multiple old publishers with a single high-tech publisher does not seem likely to be a happy ending for either consumers or authors.