Amazon has been extraordinarily successful delivering shareholder value by plowing profits back into growing their business rather than via dividends or share repurchases.

The buyback might seem to be a strategy change, but it is "only" a $10B authorization, and the last authorization never resulted in more than half of the purchasing happening:

  This stock repurchase authorization replaces the previous $5 billion stock repurchase authorization, approved by the Board of Directors in 2016, under which the Company had repurchased $2.12 billion of its shares.

Source: Amazon Form 8-K: https://www.sec.gov/ix?doc=/Archives/edgar/data/0001018724/0...