The value prop is the same for them as it is for any other company choosing a single cloud: it unlocks a ton of services and basic building blocks which empowers and speeds up software development, as well as providing fundamental stuff like access control and auditing.

The concern over vendor lock-in and pricing is one that gets thrown around often, but doesn't really make sense to me. When has Amazon, Microsoft, et al gouged one of their larger customers in the past? Why would they do that when it would destroy their reputation and scare away business?

A large customer working with a large company is like any other negotiation, both have leverage and there isn't any feasible scenario where Amazon or Microsoft go completely crazy and start gouging their customers for some short-term gain.