You are connecting unrelated dots. While it could be argued that the "cloud provider" space is an oligopoly (though I would not argue that), simply deciding to not serve a specific business, even as group, is not an antitrust issue. No business has a "right to service".

Antitrust is more concerned with anti-competitive behaviors: pricing-fixing, group boycotts[0], buying a competitor with the express purpose of removing competition, etc. Amazon and Parler are not in the same market - they are not competing for anything. Antitrust is not going to be a fruitful legal avenue.

If Amazon were somehow influencing Parler's ability to "do it themselves" (e.g. preventing a seller from providing them with boxes), then yes, there may be some legal standing.

[0] Look this one up. It's not what you are thinking.