Is it at all legal for Amazon to selectively disable it's services for no real reason? I'm surprised that it looks like this.
Even if there were some non-discrimination law that touched on this sort of situation, it's not inconceivable that they view Unglue.it as competing in their core market. I can't imagine many cases where a company would be legally obligated to provide services to support a competitor. (Sure, local phone utility companies are in that situation, but that's for solid historical reasons related to government backing of their monopoly on infrastructure.) [I'm obviously not a lawyer or an expert on these things, so corrections are welcome.]
For instance, I have a coworker who keeps insisting that Apple should have been hit with an anti-trust suit like MS since they only allowed the Safari browser on iOS (for a time). He was dead serious (for some reason he truly hates Apple and wants to see the corporation fail, still owns an iPhone though). It was based off a flawed understanding of the laws and the markets the laws apply to. A lot of people seem to jump to the legality of the actions of the group in charge of a particular platform without considering whether that platform is one of a larger marketplace.