there's no "they", there's only us. AWS paying to use it means we are paying to use it. Between that and a truly Apache 2.0 project, I think it's a no-brainer for everyone.
More generally: why should AWS get to leverage its near-monopolistic position in the SaaS market to perform essentially hostile takeovers of the software projects it packages and runs? Simply because they have the resources to do it?
The code was open, and the company decided to close it for the new versions, so it forks. Forks are common when people don't like the corporate direction of an OSS project - It's a strength, not a weakness. Openoffice/Libreoffice, Hudson/Jenkins etc.
Imagine if Linus said that the next version of Linux would be paid-only.
Within 15 minutes there'd be a new librelinux repo that people could contribute to instead.
Someone was going to make a new fork anyway - This is Amazon putting their money where their mouth is to fund and support that new fork.
Would you feel the same if AWS built a drop in/api compatible system from scratch?