This
case with Sportsbet is another example that fits quite well into this discussion. And it shows, what a powerful tool KYC can be and how gambling sites can use it to their advantage (not claiming that Sportsbet did it here).
There is multi-accounting and multi-accounting, this is the bottom line. If you are multi-accounting, but always lose all your money, then you are allowed to multi-account. If you are multi-accounting and winning, then you will be a victim of T&C's.
I am 100% sure that every casino/bookmaker knows about a lot of players doing multi-accounting at their sites - and they are fine with it, as long as it's giving them profits. If these players start to win, hit a bigger parlay or request a withdrawal, they can always user their power-tool, so this is a worthwhile, nearly risk-free, business, where you can never prove the operators being guilty.
Even if you choose to voluntarily do KYC once you opened your account, you are not safe from running into trouble in the future.
Personally, I don't see any great harm to an online casino if someone uses multiple accounts. If they are afraid to spend money on additional welcome bonuses, then let them stop giving them out at all.
All these rules and prohibitions often complicate the life of honest players. It will not be difficult for cheaters to connect a VPN or buy someone else's documents in the darknet to pass the KYC.
Therefore, when the casino leaves the rules that they have the right to ban multi accounts, it leaves itself a loophole to ban the player it does not like. Not all casinos use it, but players should be aware of it.