... n/a stands for "not 'alf!", and is a positive affirmation of the frostfreeziness inherent in that particular item.
Seriously though, marketing folks hate the presence of big fat crosses on their products and to them "Not available" is a friendlier concept than "No it bloody doesn't come with slimline salad dressing".
If you want to sell something, slap a load of "A"s, ticks and smiley faces all over it.
Don't forget to substitute "B"s for "A"s on the middle of the range model so that those who think they remember hearing something in "skewl" about the first two letters of the alphabet being in that order can work out why the midrange one is a bit cheaper.