Be smarter than a bird

I sometimes pass a hair salon in North Beach when I walk 1st Brother to school that has this sign on the door:

It's hard to see, but it says, "CAUTION Please watch out! Glass door is TRANSPARENT!"

What brought about the need for this, exactly? Have several people walked smack into the glass thinking it was an open store front? Because you know, San Francisco's tropical temperatures make that a great option (being sarcastic here). Maybe it's some liability thing? Hilarious.