Why did he buy the horse? Because he can. Why the heck would any hollywood star buy a ferrari to go get groceries? or a latte? Because they can.

I don't see the Pre Export Quarantine or Post Arrival Quarantine requirements being to big of an issue, just taking into consideration the horse he bought and that he is prepared to, at a minimum, pay for shipping. Maybe he hasn't looked into it requirements yet and didn't realize. He sounds like he really likes her and just doesn't want to leave her behind while he's gone.