Sometimes people can run into trouble with casinos simply because they forgot to turn off their VPN. This is because they may be regular VPN users in their daily lives due to restrictions and privacy concerns in their countries and they could be flagged as bad accounts just because they forgot to turn off their VPN when accessing the casino. I think casinos need to be more flexible in this regard. I don't use or support casinos that enforce strict VPN policies. Fortunately, there's no such issue at Spinly AFAIK.
True. An accident happens. I think it happened to me a couple of times. I noticed that my VPN is ON only when the casino shows I am accessing the website from a restricted region, and they do not accept players from my country. I then turned off my VPN and reloaded the page to reaccess the website. As you may know, not all casinos show the pop-up warning. So I don't actually know if I have already signed up for some casinos by using a VPN. I think casinos that requires KYC can actually allow players to use VPN and they can ask for a KYC if they suspect anyone.
It all depends on the casino on what they want from players. Some KYC casinos can allow players to use VPN to a limit but these casinos are very rare. Most times they allow VPN to attract more gamblers to use their website, when they get what they want they can update their terms to stop users from using VPN.
I would say that casinos that allow VPN are not rare as you said, I believe there are some casinos here on this forum that doesn't have issues with their users making use of VPN to access the casino so long as the user is ready to submit all the required documents for kyc verification anytime such requirement comes up.
This casinos do not have issues with VPN usage but its simply not something they think is valid to include in their marketing infors and ads, and this is possibly because most gamblers are not really concerned about VPN, they are more concerned about KYC, and this is why we have several casinos coming up with the "non-kyc casino" as a line in their marketing ads.