The Oakland Athletics have the second-worst attendance rate of any team in Major League Baseball. They average 16,159 fans per game, meaning more than half the seats at Oakland Coliseum are often empty.
Starting in June, fans will be able to purchase a monthly ballpark pass — good for admission to any home game that month, as long as there's no sellout — for $19.99.
The cheapest individual-game seats available for A's games are $15, according to the team's website. So for just five bucks more, a diehard fan could attend all 15 home games in June. That comes out to $1.33 per game. Not bad.
http://www.latimes.com/sports/mlb/la-sp-oakland-athletics-monthly-pass-20170512-story.html
The Cardinals do this, too, apparently. But for $29.99 a month. http://m.mlb.com/cardinals/tickets/info/ballpark-pass
And the Astros. http://m.mlb.com/astros/tickets/info/ballpark-pass
Brewers http://m.mlb.com/brewers/tickets/info/ballpark-pass
Tigers http://m.mlb.com/tigers/tickets/info/ballpark-pass
Marlins http://m.mlb.com/marlins/tickets/info/ballpark-pass
White Sox http://m.mlb.com/whitesox/tickets/info/pass
So multiple teams do this, just different pricing and similar structure.