Key difference:

- "No shirt, no shoes, no service" is a rule that applies to all customers, same with blocking customers that develop toxic behavior.

- Specifically targeting and dropping service for someone who launched a different competing service, for that reason, is anti-competitive.

I don't think that is anti-competitive under the legal[1] interpretation, but I could be wrong. My understanding of the legal precedent here is that a company is allowed to disallow service for any reason whatsoever as long as there is no contract in place. The only time it becomes anticompetitive is if a product prevents users or customers from being able to access the competing product.

The example I'm thinking of is Microsoft with Internet Explorer, where users were unable to use a browser of their choice. Here a company is not being allowed to use an API, but users are not prevented from using the product if they'd like. They just won't get all the functionality of Facebook within that company's product.

Facebook's behavior seems more comparable to Apple disallowing use of privileged iOS APIs in competing mobile browsers rather than Microsoft not allowing users to install competing browsers. But I'm not an expert on monopolies, so I'm happy to be corrected here.

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1. NB - I'm not making a claim or observation about ethics or what ought to be, I'm asking about what is, legally.

From [1] "Price discrimination is made illegal under the Sherman Antitrust Act. 15 U.S.C. §2, the Clayton Act, 15 U.S.C. §13, and by the Robinson-Patman Act, 15 U.S.C. §§13-13b, 21a, when engaged in for the purpose of lessening competition, such as tying the lower prices to the purchase of other goods or services."

I'd gather that the same might apply to sale/no sale and not just prices, i.e. you can't not sell something to someone purely for competitive reasons, there has to be 'some reason'.

[1] https://definitions.uslegal.com/p/price-discrimination/

Full disclosure: I'm definitely not a lawyer.

That stuff was reinterpretated in the 70/80s. If it doesn’t raise cost to the public, it’s not a problem.

Not in the UK or EU - you don't need to have market dominance to be fined for anti-competitive behavior.

We're not talking about 'toxic customers' who don't obey universal rules.

So that's not a good example.

We're talking about a store arbitrarily discriminating between customers on terms of it's own choosing.

... which is what makes it harder.

As for your 'software sold after the fact' ... welcome to iOS / AppStore to see who 'controls' what :)