It's pretty common for Fortune 500 type companies to ask for all sorts of intrusive terms they perceive as a benefit to them. Indemnity, long net payment terms, penalties for missed SLA, etc. They could be specifying AWS in this case to reduce latency and/or avoid the internet as a dependency. Maybe the client side is on AWS already, or they already have high bandwidth / dual path via AWS direct connect, and don't wish to do that for a different provider.
The reward being that once you're in, they are too big and slow to ever move away from your product.
The reward being that once you're in, they are too big and slow to ever move away from your product.