Without the need to reference insurance cover or various rules put in place over the years, limiting the event to zeds is a common sense requirement for an event such as this. But I do completely understand your question Dan and its one that is asked every year.
Wales is always a popular event that is heavily subscribed and involves considerable planning given the number of people involved. Limiting the event to zeds is therefore common sense. However, this year the numbers are less so one could argue let other cars in. But in its simplest form Wales is designed as an event exclusively for forum members with zeds. Allowing other cars moves it away from this.
In its current form it is zeds only for this year. But it is fair and right to have the discussion on the forum to see if that should change going forward, but then it would become a different event. Ultimately we are trying to ensure Wales as an event continues on each year but if that format changes then so be it, that could be a good thing or possibly not, it may have to change with the times, but this has to be planned, agreed, organised and supported by those concerned and those organising the event.
Hope the above makes sense in my rambling sentences