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It's actually not uncommon for these types of places to be posted 51%. The rationale is that the bar area may be considered a separate business, so it's not far-fetched to think that the bar may be a 51% establishment. Since alcohol must be consumed on the premises it was sold, these venues typically arrange for the entire entertainment center to be considered part of the bar's premises. The upside is that this allows patrons to take their beer back to the bowling lane rather than having to drink it at the counter. The downside is that the entire center becomes off-limits to CHL's. This sort of thing has been a topic of discussion in other threads, and there may be others who can shed more light on it.

The OP makes a good point though. It's up to the owner to make the sign conspicuous, and it's on us to keep our eyes open. Even if you "know" it's not a 51%'er, you still have to watch out for 30.06's at any location.