I guess I don't know for sure, but here is how I think(?) it works.
Player has 4 friends/family coming to game tonight. He logs onto computer in clubhouse with his on MLB ID, enters his request for 4 tickets. Based on home club pricing, etc... (sometimes including "dynamic pricing" which can be quite expensive), tickets are issued with a 'price attached' that is normal for that game, that venue. The value of the ticket (could be $100 or more...see Yankees/Mets games as a "more" example) is added to the player "income" and therefore taxed come tax day.