Remember that the 'no glass bottles' by-law is not a ban on alcohol but on glass containers. This is also not new (despite what you may read on PH and elsewhere) it has been in place for at least a decade. However, the French authorities apply the rule rather sporadically.
At motorcycle events like the Moto GP last weekend or the Moto 24h for instance, the rule is more strictly applied, and car searches are mandatory. Alcohol is also banned at Moto events.
Early in the week of the 24 hours, when folks are turning up at a manageable rate, track security will probably search all (or most) vehicles, but by Friday the numbers coming and going makes searches impractical. But... as usual where the French are involved, you can never predict when some bored official might decide to randomly enforce the rules.
As far as I know, the legally enforcible by-law applies to the general enclosure of the race circuit and the 'Zone Vert' campsites within; basically anywhere you need a EG ticket to get into.
The post race clean-up operation on the circuit campsites and trackside areas is quite significant, and broken glass is a constant hazard during race week. The ACO are (understandably) just trying to reduce the size of the problem by reducing the amount of glass coming on-site.
