We have had this discussion before and it is possible the entire fair grounds are a 51% venue. The problem is that TABC will issue one license for the address and the entire address is covered by the license. If the contracted beer vendor, for example, gets a temporary license for the weekend, his license will cover the whole property UNLESS he deliberately marks off some of the property. This allows patrons to buy an alcoholic beverage and walk around the grounds drinking it. If the vendor is making 51% of his sales from the sale of alcoholic beverages for on-site consumption, then it is properly posted as 51% for the venue.
The questionable part of this posting is that the fairgrounds are mostly outdoors. The law (Penal Code 46.03(a)(7)) refers to entering the premises. PC 46.03(c)(4) says it is only the buildings and does not include any of the walkways. Is the beer tent or booth a building? What about some of the rides? Seems to me like the posting doesn't apply until you enter the building, so don't use the restroom while you are there (the permanent ones, not the port-a-potties).