(Image credit: mobuco)
Quotation marks are used to denote a quote. Over time, journalists got into the habit of using quote marks to connote something they themselves would never say, but someone else did, implying that the journalist does not believe it. This use led to the rest of us reading short quotes as something you shouldn't believe, or sarcasm. However, people who make signs often use quote marks for emphasis, which ends up conveying the opposite of what they intend.
(Image credit: panicatthepharmacy)
Sign makers: you can make certain words bold, italic, or even underlined for emphasis. Don't use quote marks, or you'll end up in a post like this. See 17 examples of misguided quotations at Buzzfeed.