There is a trend in Games Workshop store of disallowing drop in gaming to occur there. They are requiring that you book your table in advance, to keep the tables free, as the stores are looking to become much more of a recruitment center for new customers. This means keeping the tables open to allow learning games.
Now the above is just a rumor still, but seems apparent with GW stores dropping down to 1 man shops. This is being seen in the US, so those people with bigger stores elsewhere are among the lucky. Of course we have had this discussion before, as it allows Games Workshop to open up these 1 man stores in places that have never had one, thereby expanding the hobby into new markets.... like my own here in Portland Oregon.
So before you just drop in to your local Games Workshop store for a pickup game, please give them a call.
Gadget Wargaming on Twitter has been discussing this new trend of disallowing drop in gaming, and the rest of the information is all from past discussions here on the site.. like when the Portland GW store was opened here. Thanks ieuan for bringing this to light.