Here's the deal. It WAS free. Whether it was a mistake, I don't know, but for about 4 months it was labelled as free on the SW website.
In Feb/march this year (when they brought out the new book) they decided that it should not count as a free food, and have listed it as (IIRC) 1 syn per tbsp. It is the same for all supermarket rf egg mayo's.
Remember that if you make your own, 1 tbsp extra light mayo is 1/2 syn, and you could add several eggs to make much more than the shop bought at lowers syns