On the contrary, I don't see a similar desire to contribute back from Amazon, Apple, Facebook or Netflix (although the last 2 do contribute a ton to OSS).
My response to this is: so what? Of course they're going to encourage new developers to use their products - it's only natural they'd teach their own stuff and not AWS/Azure. Developers aren't really influencers (sadly) in the purchasing for cloud infra so I'm not sure that it's a straight up marketing campaign.
It’t a great hiring strategy, as many researchers prefer to work at a company where they can get recognition from publishing academic papers.