[zamadatix]

The EU's regulatory stance on antitrust does not require a monopoly, it requires a dominant position in a market meeting use of certain criteria marked as abuse. https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CEL... From there when they tell a company they are breaching criteria for abuse and the company doesn't change the EU issues according fines.

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).

[simondotau]

But Apple doesn't have a dominant position in the EU.

[zamadatix]

They do, in several areas, at least according to the definitions of EU law. This may not meet your individual definition of dominant position of course (e.g., one might hold you need 50% market share in certain markets to be dominant, but the EU definition does not hold this requirement).

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.