Many public datasets are published under licenses that require attribution, such as CC-BY. If I have a public SPARQL endpoint, and allow federated queries to access data from such endpoint, that means that a query result can contain data that is under CC-BY. However, as far as I understand, there's no standard way to incorporate attribution into SPARQL results, even not counting the hassle of actually figuring out the fact that federated query is being used and what kind of attribution each of federated endpoints require.
So, the question is - what is the best practice for complying with attribution license in such scenario? As the endpoint is public, the combined data can be reused anywhere (so it can plausibly be considered redistributing?) but I see no way to ensure CC-BY compliance (unless I misunderstand its requirements). Does it mean that attribution-licensed data can not be feasibly used in data federation scenarios? Or does it mean attribution requirement does not apply to such scenario, or should be satisfied by the end user of the data (such as publishing the results of a federated query in a paper) and not by intermediary tools processing the data?