The worry isn't getting in the hole to online betting sites. The worry in the 70's, 80's, 90's wasn't getting in the hole to casinos.
The worry is getting in the hole to people who give you money, loans, that you use to place bets at casinos (then) or online sites (now). NFL or MLB wasn't concerned Harrah's or Bally's or MGM would own a players gambling debts. They didn't want their players to owe money to the people who gave them the money to place bets at the casinos. The kind of people (organizations, we can say) who would ask the players to pay back by doing certain actions in games
The worry is getting in the hole to people who give you money, loans, that you use to place bets at casinos (then) or online sites (now). NFL or MLB wasn't concerned Harrah's or Bally's or MGM would own a players gambling debts. They didn't want their players to owe money to the people who gave them the money to place bets at the casinos. The kind of people (organizations, we can say) who would ask the players to pay back by doing certain actions in games