A huge portion of the small to mid-sized clients I have which end up using AWS do so because they've been approached by sales people handing out free credits like candy. Often it short-circuits cost/benefit evaluations completely.

But AWS also often start as a $10/month product, and then one more, and one more. I think that's a huge portion of their success - it's easy to ramp up the cost slow enough and in small enough steps to not trigger any major approvals until it's "too late". People usually move off AWS first when if it gets to be enough of a cost pain-point to make higher ups take notice.