The Fire Phone was a midrange Android phone from 2014 preloaded with Amazon's app store and services instead of Google's. It failed on its own merits, but I think Google got scared someone else would do a better job. The barrier to entry is a lot higher if most people can't use their banking app on it.
Do you really think banks care about you installing custom themes or whatever? They only care because the cracked Spotify app also has malware bundled on the side which uses the new root access to break the app sandboxes.
I also consider restrictions against open source to be malicious acts - hence also malware, such as by banks and other companies. Or, worse, government agencies. In a local medical university in central Europe, for instance, we are required to either have a Google account or a Microsoft account - usually the latter, to access emails.
All paid by the taxpayers ...