There are no absolute morals. But I think in general healthy societies are arranged around the ideas that people should have: the basics of living (housing, food, vacation, and some luxory), agency, and equal opportunities.

It should be clear that having a small number of companies murder all competition and personal freedoms (like doing what you want to do with something you own like a phone) are in contrast to these basic values.

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Or the alternative, more blunt answer: it does not require a moral justification. EU citizens directly elected the EP, the EP ratified the DMA. So Google can either comply or leave the EU as a market (which they wont do because it's too large and others would be happy to take it).