As you well know, because Musk is a free-speech absolutist/crusader. Actually so much so that he is constantly attacked, and vilified for his courageous position. He is also a bold defender of the light of human consciousness itself. So working backwards from there you can determine that deleting posts is actually a form of constitutionally protected speech which X (the everything app) is doing, as a public service for all of humanity.

There, FTFY