Ah. I wasn't thinking of those sorts of signs. I could see using quotes for emphasis there if it was really necessary and that was the only option.
And I do agree that quotation marks, like everything else, is mutable, regardless of its origin. Punctuation has changed quite a bit in the last couple centuries, and we think of this as an improvement.
I can see why people think they need quotes for emphasis or for whole slogans. I just don't think they really do, because sometimes it causes unintentional alternate readings, and sometimes it's just visually distracting. I think that in general, the average layperson overestimates the amount of visual emphasis or adornment they need to apply to text.