Certain words on the sign are being *emphasized* just like you do with a text message.
I'd instantly delete all messages plus the phone number of anyone who does that at me. I've never seen that convention either. Asterisks, sure. All capitals or using spacing too. But not this. Thankfully.
Haven't you read an Old Document like the US Declaration of Independence? Capitalization was used for Emphasis back then.
Then it got out of hand, with everyone capitalizing every other word, and it was decided to stop doing that.