Anyone who buys it or finds it in the wild can also upload it.

Proves they really did hack something. There's other sites where hackers register defacements etc.

If Troy authenticates the data, they can use that as an 'endorsement' when trying to sell it.

This. Typically HIBP attribution includes the email of the "submitter". Various data aggregators will contact them and buy the stolen data. Everybody wins*.

* Exceptions apply.

Where on HIBP can I see the email of the submitter?

It's not available in this case, or every case. When available, you can search "The data was provided by" in https://haveibeenpwned.com/PwnedWebsites

Thanks! Slight correction: only 2 breaches say "provided by" with a source, but a ton of breaches say "provided to" HIBP with a source.

Doesn't the value drop dramatically if it has already been shared with Troy and the HIBP database? Or is there a time frame where it has been authenticated by Troy but not yet added to the database?

I don't think so.

Troy isnt publicly sharing the credentials and that's what's valuable — especially having "exclusive" access.

He blogged or tweeted about this at some point. Sadly, I can't find the link.

Is there a way to modify the HIBP reporting process to avoid aiding the sale of stolen data?