Yes, if your card doesn't have a pin, then it's probably not usable on unmanned terminals. But this is kind of by design, isn't it? Like those NFC credit card stickers, that can only be used on NFC terminals, because they aren't a smart card and don't have a magstripe.

I think the operators could still charge the card without a PIN, but the card owner could dispute the charge and just get the money back. That's why they don't accept it, they want to get paid, and not give away free gas.