Data in and of itself cannot be copyrighted under U.S. copyright law: "creative arrangement" of it can be, but given that the data in question was generated from XML file, I don't think you could make a claim that the arrangement was copied.

? I don't follow - it seems pretty clear that this falls under the GPL from my reading of what is copyrightable, ie. they can't copy your compilation of the data.

Your reading of what is copyrightable may not be entirely in accord with the US Supreme Court's, which has ruled that mere compilations of factual data (a telephone directory in the case that set the precedent) are not copyrightable.

https://en.wikipedia.org/wiki/Feist_Publications,_Inc.,_v._R....

If someone is trying to apply the GPL to stuff which isn't legally copyrightable in the first place (as may be the case here), then their copyright isn't enforceable, and neither is the GPL.

There are definitely jurisdictions where this has been found not to be the case. In Australia the protection of databases is incredibly murky[1].

Telstra, the privatized government telecom company, failed to protect the White Pages data from another company that, if I remember correctly, had put it on CD (ah those were the days).

[1]: https://www.mondaq.com/australia/copyright/290668/can-a-data...

> collections of data are copyrightable as well

Collections of data are sometimes copyrightable. Depending on the jurisdiction, it may depend on the details of the collection.