Maybe unmodified AGPL software can be run as a network service without providing the source code because the source code is already available from the original author.
If you get more nefarious with it, what if they started building tooling around it -- like a proxy that rewrote requests to fix a bug, or a file system that sat underneath the program that solved some issue with IO?
I also think that the majority of value delivered by something like RDS is not actually proprietary changes, it's just the underlying software being kinda good (because it was developed in the open, and the F/OSS community was leveraged as bug finders and fixers) -- I'm thinking mostly of Postgres. Even things like HA and scalability are often considered in projects these days, so you wouldn't even need to add much to check off the usual enterprise-y boxes.