It just doesn't make any sense, why would a company that is already making almost guaranteed money decide to steal 15 BTC that could in turn hurt their reputation and possibly put them out of business?
Because they can? Why they wouldn't? Who doesn't want more money? How can you be so sure that they're making enough money? If they're making enough money off bets, there's someone definitely losing on the other side? Bet Matching, bookie chain etc are some of the concepts what I feel you have failed to understand and your knowledge in sportsbook is weak.
Because they're likely making decent money, so it would be a waste to gain a moderately sized lump sum and kill off a large revenue stream. The person losing on the other side is the bettor, as they are accepting very poor odds. I don't see how bet matching is related, but someone trying to arb on sportsbet.io is either stupid or taking advantage of promotions. Or, if you meant sportsbet.io matching bets, they could easily match with other bettors on their platform if there's a lot of volume, or even use another book such as Pinnacle, Betfair or Fairlay to counter each bet, and still make a decent percentage. There's also sportsbook providers that pay you a % of each bet, regardless of the outcome, and they could be using one of them as well.
I fail to see how a bookie chain is related to this either. They don't seem to be obviously mirroring the odds of another sportsbook like other books do (BitcoinRush mirrors Pinnacle, most Fairlay market makers seem to as well).
Throwing in random buzzwords and not explaining or elaborating doesn't make your argument any stronger.