They can't sign a contract. They can't become contractors.
Why do you keep fighting this? It doesn't even seem like devil's advocate anymore. :/
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it's anyone "under contract".
It's not. I'm not an employment lawyer, but that's definitely not true. At all. Under any circumstances.
If you need help with it, check this: https://www.quora.com/What-is-the-difference-between-a-vendo...
But further, even if it was a violation if minors were involved, that'd leave open the question of whether use by contracted adults was compliant under the terms. (And supposedly the Google app wasn't offered to minors.)
And, paid research subjects meet the legal definition of contractor, as outlined here or elsewhere:
https://dictionary.law.com/Default.aspx?selected=939
Simply insisting "definitely not true" is not convincing.
In this case, it's likely (IANAL, nor are you) that each individual user of the service would be described as a vendor selling access to their data. The data itself is not created for Google's (or Facebook's in that previous case) consumption. The users of the service were selling rights and were not producing anything for hire.
https://www.quora.com/What-is-the-difference-between-a-vendo...
https://english.stackexchange.com/questions/248665/contracto...
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This is becoming tedious. I'm out.
If the mechanism for bringing participants into "Screenwise Meter" involved a contracted payment, it plausibly matches some of the expressly permitted uses, in the Apple Enterprise terms. (If it included an express written contract that limited the participants' use of the app, it further matches certain explicit requirements of the Apple terms.)
(There's another clause about using a specific "Network Extension Framework" that seems like a bigger problem for Facebook/Google, depending on what they likely did with that API and the info retrieved. But these clauses, about "internal use" and "permitted users", seem fully compatible with an internal-research-program using a panel of compensated research-subjects.)