Maybe it's a liability issue.  Sort of like coffee cups stating that the contents may be hot.  Perhaps I should find someone to gently t-bone me at a light that doesn't have this and then sue for millions!!!

"It wasn't green, but no-one told me I should wait!"  Cha-ching!!!

But how can it be a liability issue if it's already the law?  The warning on coffee cups isn't reminding people of an existing law that says, "You must be careful while holding a cup that contains a hot beverage."  I like the idea of suing, but I don't think this is about liability.  I'd really like to know what it's about, so please try again.

Drat!  I thought I had it....I suppose I was thinking of the laws of common sense as opposed to traffic laws. 

oh, wait!!!!  Is it a context thing?  Perhaps that's a left turn only lane, and both signals aren't always green at the same time.  They probably planned on putting the arrow template in the light, but spent the last of their budget money on the snazzy sign. 

Or maybe it's for those who are color blind and can't tell which way is up!

its because the opposing traffic get a green light first.  this is one of those "delayed green" lights.  just b/c you see the other side hauling a$$ does not mean you can.  please wait for green.

The context is this:  I was sitting in the left of two lanes, one for going straight and one for turning right.  I'd say the sign is for the right turning lane, but they should've just put up a sign that said, "No turn on red". 

I'd agree with the "delayed green" idea, but it still seems kinda stupid.  A "delayed green" sign simply states something that might not be known:  The people coming the other way get to go before you.  But "wait for green" implies "not waiting for green" is acceptable in other instances.