You’re conflating different situations. Apple has some reasons for why they don’t want to make an Android iMessage app themselves and other different reasons for why they don’t want someone else making an unauthorised Android iMessage app.
I disagree with the former but agree with the latter. I would prefer if Apple made the app but I can also understand why they won’t just trust a random third-party who keeps insulting them and acting entitled to their platform to not turn nefarious or have security holes Apple has no control over.
