Because a lot of the online services that are seeing higher use (whether it's telework support, e-commerce, or otherwise) are, in whole or in part, hosted on AWS.
Yes, through accounting magic, they're making the majority of their profits via AWS, but that's part of Bezos's long-term strategy and could be changed on a dime (at the potential expense of the long-term growth he's targeting).
Top line, most of their business is from online retail, not AWS.
(edited because my original was a bit of a flame, and didn't clarify profit vs revenue distinction)
Tomorrow they could decide to use the AWS profits for growth, and reduce the growth spending on Amazon by the same amount, and suddenly the sense of your sentence is reversed.
Shareholders seem to think otherwise.
I think what shareholders value is a little deeper than woo “AWS appears profitable”!
Amazon wasn't even profitable until 10 years after its founding. So that's demonstrably incorrect.
BTW, the thesis that "if a business isn't profitable, it wouldn't be able to grow" flies in the face of hundreds of successful venture capital businesses funded by the yCombinator (founder of Hacker News) and many other venture capitals funds. Profitability is one variable in a very complex mix.
I personally like the idea of bootstrapping, which does require profitability, but that's not the only way to successfully grow a business. You can also grow a business using debt, or by using equity.
Edit: When I read his letter in 2004 I went long and I've never looked back. https://www.sec.gov/Archives/edgar/data/1018724/000119312505...
With so many people inside everyone is using online services: streaming, shopping, chatting, video, audio, gaming.
Almost anything "online" is very likely to run at least something on AWS these days.