- Uber is one of AWS's most important customers. If Amazon does a move that directly competes with Uber, to the point where Uber loses significant market share, enterprise customers will certainly lose trust in AWS. This is already an issue for AWS - Satya just gloated about it during the earnings call - but such a high-profile case as Uber vs. Lyft will only drive this point home further, allowing Azure to accelerate (which is already growing faster than AWS).
- Amazon makes no money, except through AWS. Purchasing Lyft is a significant investment over a a long time, and the money has to come from somewhere. If AWS lags during this critical juncture, this could cripple Amazon hugely.
- And Amazon has no giant cash pile sitting around like Apple. It cannot afford to cannibalize customer trust in AWS when it is imperative to their survival. See: their stock dropping, partly because their growth slowed slightly, and partly because it seems Azure's growth is faster.