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Multiple accounts hurt sportsbooks' bottom line, and help gamblers hedge their bets. This will never change, it's the nature of gambling. By adding the multi-account restrictions into their ToS sportsbooks are just trying to protect their business interests. If you're a gambler that wants to hedge-bet why not use two or more different sportsbooks?
There are definitely opportunities for sportsbooks to use KYC to selectively void successful bets, and I'm sure that does happen. I believe that asking for KYC right off the bat, however is a non-starter. Most clients won't oblige a sportsbook that requests documents before deposits, and unfortunately it's most likely because they're in violation of one of the sportsbook's terms from the begging. I'll use myself as an example: I live in a state where online gambling isn't allowed, and my favorite sportsbook doesn't offer it's services in my country. So to use my favorite sportsbook I have to travel to a neighboring nation. If they were to ever ask me for my KYC dox, I would have no choice but to disclose that I live in a country where their service isn't offered, and hope that my explanation would suffice. Of course, if they were to suspect that I used their service while in my home country (through the use of VPN, for example) they would not only be in the right to lock my account, it might be their legal obligation.
There's also the stigma on the industry itself: The gambling industry is probably the second oldest profession in human history, and only slightly less stigmatized than the oldest. It's understandable, many people find it addicting, and it can be damaging. So here comes the government, once again claiming to protect us from ourselves. Being somewhat conservative (small case libertarian) in my political views, I find government intervention more damaging. I live in a state where they allow a certain ethnic group to build casinos (it really sounds preposterous when I type it out like that,) and there's no lack of them popping up in old cow pastures and on desolate hillsides. I'm perfectly allowed to throw my paycheck into a slot machine, on a craps game, or a roulette wheel, but I can't (legally) make a much less risky bet by gambling on a football game from my desk.
Sportsbooks aren't run by dumb people, so they must know this. Which is one of the reasons they often find themselves in disputes with their clients. It's such a slippery slope; on one hand sportsbooks want to attract as many clients as possible, and on the other hand they want to abide by local laws and protect their interests.
Which brings me to the point of my post: THIS IS CRYPTO! I've never considered myself an outlaw, but there have been at least a couple of nanny-state laws at which I've scoffed. This is the freedom that crypto currency allows us; to do the things that the nanny-state prohibits with their hypocritical efforts to protect us from ourselves.
I hope you're right about some form of blockchain or smart contract technology that help resolve these issues. But until then, I think we're destined to see online casinos attempt to protect themselves with restrictive Terms of Service, and gamblers will continue to violate them.