Also not a lawyer, but cartel behavior usually refers to price-fixing and other anti-competitive behavior. Small businesses banding together the way you described would just be lobbying and already happens frequently.

Amazon should be investigated for anti-competitive practices, and their ability to negotiate this kind of a circus is certainly an indication of that. The problem has been two-fold -

1. The Sherman act is over 100 years old and was written to target 19th century trusts like Standard Oil and the railroads. These need to be updated to reflect new types of models of trusts and conglomeration (and accumulation of power) that have developed.

2. Enforcement - the SEC and FTC are largely toothless and everyone knows it. If you are a company that can afford a team of lawyers, its unlikely you will be penalized in a way that actually hurts.