I certainly wouldn't choose to continue serving a client if I thought doing so was costing me money.
As an aside, I actually would continue serving a client if I had sold them a service, they were paying for it at the agreed price and keeping on top of invoices, and if me reneging would cause them to go out of business. And then if I had to end it, I would give them as much notice as I could practically provide (particularly if they were willing to profitably pay during the notice period).
I wouldn’t personally do business with you if I thought you would instantly back out of an agreement that you had already profited on with almost zero notice. AWS is no charity here, Parler were paying them for servers.
Are you trying to say it's unfair? I just don't see it that way. If Amazon violated the terms of their agreement then fine, let it be litigated. But if Parler can't get back up quick enough in the mean time that's on them. That's how business works. I know. I conduct it. It's not personal.
And oligopolies pose antitrust issues.
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.
Parler had no idea AWS would just yank them from service at 24 hours notice - I don't think there is any precedent for this from an AWS perspective. So this is the challenge they had if they wanted to move to physical infra after being given notice - 500 servers in 24 hours.
(As a side note, setting up physical infra at the scale of growth Parler was having wouldn't have been practically possible - they had a giant and rapid spike across the last month or so. It also wouldn't have been financially wise, considering they probably don't have the capital and the traffic spike was probably a temporary surge. Plus if you buy physical infra and then your apps get removed by the other cloud providers, that changes your traffic profile again... Parler really had both their hands tied behind their backs to fight this thing).