They're not authorised. They provide a free service here at tPF, based on years of collecting and an interest in the brand. They don't claim to be 100% correct all of the time, but they work together and each of the authenticators has different areas of expertise. Many people here including myself have relied on them to save us from nasty eBay fakes!
LV officially do not provide an authentication service. That is not to say that an SA might not offer their opinion, but they will not officially authenticate an item in store. The SAs aren't trained to do that, they're trained to sell. To be frank, some of them have very limited product knowledge and I wouldn't personally trust just anyone working in LV to tell me if an item I purchased online is genuine.