You also never hear anyone complain about the thousands of hosting providers that build their business around the LAMP stack. I wonder if it is because Amazon is so big, or just the lack of competition.
I think it's a general sense of unease with the probability that future F/OSS looks like "{Project}, sponsored by Amazon" due to cloud market dynamics and size.
Coupling that with the fact that, in the past, F/OSS project control or domination by a single party hasn't turned out well.
They sell a hosted service that uses the software. They're not selling the software.
Pretty difficult to see how a company building a SaaS business around some software is not "for their own use".