Just because Wal-Mart as a coporate entity knows about the date being moved forward doesn't mean their staff is aware... Keep in mind this isn't
Gamespot Gamestop or any other video games retailer, we're talking Wal-Mart (there's a reason many french speakers here in Quebec call it Wal-"Marde", litteraly Wal-Sh*t...) here so the staff is not necessarily all that dedicated about their job.
You also have to keep in mind that Brink still is a pretty low-profile release and has built very little hype, all things considered (compared to, say, L.A.Noire, which was most likely seen as
the May 17th release - and possibly largely responsible for the Brink release being moved forward!). Hell, the whole Amazon.ca team is unaware that Amazon is supposed to offer a preorder bonus, I can't expect every single Wal-Mart employee to know Brink even exists

As for their preorder policies, I have no idea (since I
never deal with Wal-Mart in any way; you can think my convictions stupid if you want, I won't mind

), but my first instinct would be to go in store. If the rep says no, ask to talk to a manager, and mention that the sales rep refused your purchase and you'll have to go to another store to make your pre-order. I'm pretty sure they'll change their mind

+ Edit : Oups, typo...