For me, exploitation has always required the exploited party not having a choice. Customers of AWS ES vs. Elastic ES definitely have a choice in this matter and have done so deliberately.
Due to the market share of AWS for many potential customers it will come down to a decision between
a) click here in the aws marketplace and have my service added to the invoice my company gets every month anyway
b) start the painful process of onboarding a new vendor, get a quote, have procurement haggle with them for months etc etc etc. (overstating it a bit, I know)
And that is where to me it becomes a question of using/abusing your position in the market to a certain extent..
The rest of the 500+ comments here do a good job arguing both sides of it so I won't rehash that. Just saying that I can see that AWS taking the free thing and using it as a loss-leader (i.e. no possible way Elastic can compete) could be construed as exploitation of the free thing.
Even if it is though, I'm just not sure I find the idea of free software being a loss-leader a bad thing. Having a loss-leader implies that AWS is providing + capturing value elsewhere. In this case, you assert this is from Data Transfer. But regardless of where the cost-centers are, AWS is delivering value to their customers and charging for it. There is competition in the cloud space, and in all honesty moving clouds generally isn't a huge deal, at least if you've architected your systems with that in mind (which any SysAdmin worth their salt should be doing in 2021).
The fact is, customers like having all their cloud managed under one roof. Customers like the exceedingly strong uptime guarantees provided by AWS. If customers have choice (which they do), and they're choosing AWS, why is it a bad thing if Elastic can't compete? If AWS is winning, it's because they have a holistic offering that customers like better.