This is standard for most casinos and sportsbooks. Sportsbet is no different in this regard. It's not "fair," and it certainly benefits their business more than if they were to demand KYC from the getgo and check for multi-accounting before betting began, but good luck finding a sportsbook with competitive odds that doesn't do this, and let me know when you do find one.
No worries, I do not need to find the "perfect" sportsbook, as there is none.
Nonetheless, I have seen many other companies that accussed a player of multiaccounting and shut their account. But then, they reimbursed the WHOLE account balance as it was when the account was frozen, no exception. And not after one week.. mainly after 24h, only.
My sole mistake was to trust Sportsbet.io ethics. Nothing else.