I'm 99% certain that they mean that your whole face has to be covered, not that it has to be covered by a full face mask, but yeah, phone 'em. It's right that you'll get hit sooner or later anyway - I've only been into airsoft a few months and I got hit in the mouth through a tiny hole I've made in a Viper Comforter lower face mask. It's literally just wide enough for a roll-up to fit through, but I ended up with a BB in my mouth! Another time I was playing in mesh goggles & half-face mask plus a MK6 kevlar helmet - I got drilled in the forehead through a tiny gap between the goggles and helmet rim!
I think you've got more flexibility in getting a good fit with separate goggles and half-face, but having said that I've just bought a Killer hard plastic full face with mesh eye holes which fits really well, its just that I won't be able to wear my helmet with it. Haven't skirmished it yet though so I dunno what if any other problems it may present...