For one thing, they’re unlikely to be regularly changing out the tape on their pistols. It’s also very possible that in an actual conflict weapons could be picked up by the other side.

This is more for “Oh, that guy is dressed like a fed and his gun has red and white striped tape on it. He’s DSS (Diplomatic Security Service). I’m Capitol Police, so he’s probably not going to hear me if I call for his attention over our internal channel. Better transmit on the shared channel instead.”