An upstream contract that required detailed information about paid users, for instance? I could imagine wanting information like that for auditing if there were complex restrictions on permissible resale of the upstream data.

"Honest curiosity though: What kind of contract would make it impossible and would one make such a contract? "

Most of them :) They often have requirements on downstream users, such as requiring we provide info about them, or that they be restricted to certain use cases, etc

There are a lot of ridiculous restrictions i've seen in my days. Most data brokers don't really want customers to go through you to get "close to bare data". Thus, they give onerous contracts, and restrict how you can sublicense the data, as well as usually requiring attribution, in the hopes that people will see your restrictions, see the attribution, and buy the data from them instead.