Because they are designed to attract new business, not just reward people who already know who you are and buy from you anyway.
Limiting it to 'loyal forum members' only would somewhat defeat the point of the marketing benefit this sort of thing brings.
It'd be a wasted opportunity if they were to give away £1100 worth of stuff with no possible benefit to them beyond a few forum members thinking 'wow, OcUK rock'. Open it to all and you get a huge number of people looking at your shop website that may never have seen it before, it gets cross posted across the internet possibly reaching people who have never even heard of the company etc. etc. and may just drum up a tad more than £1100 worth of business from new customers that may otherwise have shopped elsewhere.