Does the Marlins trading away their entire team constitute tampering? If BGS demands a trade, it's not because of Bryce.
The Marlins trade constitutes something very evil which I find hard to characterize but "tampering" doesn't describe it.
Nevertheless, I think that Harper encouraging Stanton to come play with the Nats is tampering for the following reason. Say what we will about the Marlins move, they didn't do anything against the rules and presumably there is a business plan behind it. One component of that plan is to keep Stanton as part of the team. Whether that's so they can get a better price when they sell, or because they want to rebuild around him, I don't know. But by deciding to keep him, they want to keep him happy. At least, keep his level of disgruntlement to a minimum. Harper encouraging him to want to be traded only makes him more disgruntled then he already is. To me, that's tampering.