Doesn't revealing entail putting your maneuver dial face down on the table and then, well, revealing it to your opponent? Whereas SLAM is just asking you to pick a maneuver on your dial and execute it. You're not placing down the dial and then revealing anything, you're just looking at it and telling your opponent, "Okay, I'm doing this one."
EDIT: In other words, "reveal" is a specific game action and not simply a synonym for "show," "choose," "select," etc.
Edited by DailyRich