Why is is against the law?
As for "why" this is against the law, I assume that more to mean "why did the EU make this against the law" (since the other answer is simply "because the EU law was written as so". The arguments are largely the same as for why monopolies should not be allowed to operate: to ensure free market competition by preventing a few dominating companies from unduly pressuring the market. There are, of course, some who feel the freest market is one with no governmental regulations at all but they are not the majority (at least in the EU).
They also meet the definitions of a gatekeeper (as defined by the DMA) in several areas, which is the related law this fine actually came from.
Analogies aside, the REAL question is whether Apple is entitled to charge money for access to their developer APIs. Or whether Apple is entitled to place software license terms upon use of their intellectual property, e.g. when you link against Apple libraries which are then compiled into your binary.
We get up in arms about GPL violations, but also want Apple to suck shit. I don't think it's right to want it both ways.
The people of EU decided it will be law that Apple must allow for alternate app distribution means - you must be able to side load and/or install alternate app marketplaces. That is the law, like how other countries have different laws for parental leave, for example.
The EU regulators have found that Apple has not complied with this law because it makes using alternate app marketplaces purposefully unattractive and burdonsome for both developers and users. EU is clamping down on Apple's 'malicious compliance'.
I am a hard-core fan of the GPL and I recognise that the GPL license requires intellectual property rights in order to work. You want copyleft worth a damn? You need copyright. And that means you get copyright. Apple has intellectual property rights over their software and that doesn't give anyone else the right to "do whatever they want" with it.
If you want to cancel all intellectual property rights with respect to software, that's an interesting argument to make. But cancelling it under a few rare circumstances when some software irritates you seems like the height of absurdity.
As for IP license rights over all other law, I think most people are in alignment with a circumstance basis (or, more succinctly, "local law has priority" basis). E.g. most are indeed happy to declare a software license claiming women are not allowed to distribute the software is invalid in the jurisdiction - a company's "IP rights" be damned over a person's human rights, as an example.
Would you say that's pretty discriminatory?
(Analogies aside, while I understand their rationale, it was wrong for Apple to lump the likes of Spotify and Netflix within their in-app purchase umbrella. And I also find it hilarious how people think 30% is highway robbery when 10+ years ago it was widely regarded as a fantastic deal for developers.)