Nothing is large enough by itself to alleviate climate change. We can all cleverly rationalise why this or that action has little effect and watch the world burn in our smart inaction. Or we can do our bit where we have some influence.
This is what I dislike about activists. The whole "any action is better than in action" attitude. Actions are only good if the positive value they produce outweighs the damage they cause.
For example, if we were to shut down all of industry everywhere, we might reduce our fossil fuel usage to a minimum, but it will not help the millions of people who would die in the aftermath of such an action.
In this instance, insisting that Amazon kicks oil and gas companies off of its cloud services is an empty gesture. There are plenty of alternative services (Google and Microsoft come to mind and they are not the only ones) the infrastructure re-build cost will be absorbed into the pricing model and in the end nothing will be gained and you will pay for the change with your heating and gas bills. But hey, at least you can wear the "I did something" t-shirt or whatever