Probably to show that it's disabled.
Also, at 4:28, Shishka's talking about Invasion having more varied layouts than Slayer, and they show (what's probably) Elites/Covies Slayer with two layouts of Warrior (Plasma Repeater+Plasma Pistol) and Zealot (Needler Rifle+Plasma Pistol).
Is this going to be the standard for just Elites Slayer, or is Spartan Slayer (or Team Slayer) going to have something like DMR+Pistol and AR+Pistol to choose from, or just the same weapon combos with different armor abilities?
Probably already asked/answered, though. :/