>But this is about blocking payments to the VPN providers, which seems somewhat different. Are they worried that the purchase of VPN services is itself often done via CC fraud?
Yes. the nature of credit card "security" is that, well, there is no security. The agreement between the merchant and the bank is that if anything goes wrong, the merchant has to return the money.
This means that they generally only want to give merchant accounts to people who are "good for it" - they will check your credit, and if it isn't awesome, they will either hold back a bunch of your money, or deny you outright.
They want to avoid industries with a lot of chargebacks for the same reason.
"adult content" has the same problem... you get a /lot/ of chargebacks, so if you want to have a merchant account and sell adult content? well, you are going to pay substantially higher fees, and you are going to have a limited number of providers who will take you at all.
But yeah, from what I've seen, reputable VPS providers have fairly low chargeback rates. I've never run a VPN service, so I wouldn't know, but from what I have been told, the chargeback rate is way higher.