Make no mistake, they do these bullshit investigations regularly for anyone trying to cash out, at least asking for ID documents, utility bills and the likes again. And not just P*. An attempt to pursue the masses to keep their bankrolls on the site and keep playing, rather than cashing out? Probably.
But this here... I think it's two-fold. Reads for me like the guy pressed his luck and played from the US via VPN, believing presenting his second residence in Canada would be enough to have everything look okay on paper for everyone. P* would be out of any potential legal trouble; P* would be happy to have another player on their site generating revenue, he would be happy being able to play poker online, win-win?
But P* asking for fucking proof that he was physically in Canada while the tournament was ongoing... how the hell are you supposed to prove that retrospectively? Present his neighbors as witnesses? Wtf.
Is PokerStars, the by far largest poker site in existence, really that desperate to risk incredibly bad publicity (less players, less revenue!!) just for a shot at bolstering their profits a bit by scamming a player out of his rightfully won money?
Sad times.