Indeed there are no reasons that anyone in the US or other restricted countries cannot play from the faucet and withdraw their winnings.
Many people in the Bitcoin and Crypto world like to protect their privacy using VPNs. I don't know of a casino site that prohibits their use or monitors if customers access from another country, say if they happened to go on vacation.
I always wondered how the legality of "monitoring" your clients is laid out for countries that do not have KYC and similar laws/regulations. KYC forces you to verify your customer to a certain degree, verify their location, identity, etc. However in these other countries that do not enforce any kind of KYC-like legislation it makes me curious how their client-base is regulated at all.
For example, the company that does not have to Know their customer but is also not "allowed" to serve US clients has almost no way of verifying whether or not they are serving US clients; other than simply going off of current IP, which is not a verification at all; it can be circumvented, it can be manipulated, as you said people can go on vacation or even immigrate. There may be some clients that are allowed to play, but wish to not display their location and are refused because they are using a USA IP.
It would seem to be the case that US clients are only artificially restricted until there is KYC enforced worldwide or there is a more stringent validation than current IP. Although, like any business there is incentive to bring in as many customers as you can so I don't see the casino's stepping-up to the plate just to alienate an entire country full of gamblers (especially when they disagree with the KYC doctrine, anyways).
tl;dr Are US Citizens not (legally) allowed to play or is there just restrictions on US IPs? I'm a little confused on the legality.