PayPal might've just realized they're a competitor.
If Masterbucks is "like PayPal" from that perspective, subject to certain de minimis thresholds, then they too are likely to require registration.
Frankly, if this is the case, and the Epik people are complaining that PayPal's due-diligence questions on these topics are "absolutely absurd, and well outside of any knowledge or experience [they] have as a domain registrar" then perhaps they oughtn't to be running that kind of a business?
Yes, because it's using their payment network. I'm not sure why your example involves Visa auditing a purchase of a prepaid mastercard.
PayPal is the middleman that turns Masterbucks into cold hard cash into your bank account. If PayPal didn't enforce it, then in the eyes of the government PayPal is complicit also.
Your example is not really the same. If you started using your merchant terminal to take payments on your own off-brand money transfer service, you can be pretty sure that you're going to get cut off just as soon as your processor finds out.
The currency they're selling purports to be a general-use currency for real life sales.
If you allow transfers between users and cashing out to real money (as Epik's "Masterbucks"), then you'd have to comply with these kinds of regulations, so video games try and take steps to ensure that their currencies aren't treated like that, or at least if this does happen, they can't be seen as facilitating it. Epik's marketing material explicitly mentions both "fully transferrable" and "convertible to dollars" - this combination means that you're intentionally running a service that can be used for money transfers, and thus you need to ensure KYC (know your customer) and AML (anti money-laundering) compliance or shut down that service.