The most likely explanation for Amazon changing its approach is that the issue was taken over from risk-averse managers who hoped Parler would do the right thing by closure-hungry executives who were not willing to wait and were willing to risk a lawsuit and conservative backlash.

The more interesting question is why Parler were willing to risk being shut down in this way. I see two possibilities: either they did not have a realistic understanding of their situation, or they preferred to have Amazon pull the plug rather than being responsible for unpopular moderation of their users.