On the Apple Developer Enterprise Program webpage [1], the first big feature (of four) is "Deploy In-house Apps" -- "within your organization", "to your employees' devices".
I'm not sure how it could be much clearer that this is not intended to be used to distribute apps to customers.
But the people using these research apps are not, in their role as app-users, "customers". They're contractors, being paid for a service rendered. And other provisos of the agreement specifically define "Permitted Users" to include "contractors".
I'm fairly sure that neither Google nor Facebook actually want to consider them contractors, as that would likely mean that Apple ToS isn't the only thing they broke.
I'm not sure how it could be much clearer that this is not intended to be used to distribute apps to customers.
[1]: https://developer.apple.com/programs/enterprise/