Those signs were carried by leftists, not TEA partiers.
You don't know that.
Actually, yes we do. Below is a newspaper article that detailed the organizer of the effort. The website link is now disabled but if you care to look around you can find out for yourself that it was indeed lefty's who were doing that.
"Sabotaging the Tea Party
In the age of the Internet, every action has a disproportionate and outrageous reaction.
See, for example, the toxic -- and tactically stupid -- response to the rise of the Tea Party that one activist is cooking up. Jason Levin has dedicated himself to destroying the movement, and part of his strategy is publicly declaring that he and his allies are infiltrating the Tea Party:
Whenever possible, we will act on behalf of the Tea Party in ways which exaggerate their least appealing qualities (misspelled protest signs, wild claims in TV interviews, etc.) to further distance them from mainstream America and damage the publicÂ’s opinion of them. We will also use the inside information we have gained in order to disrupt or derail their plans.
As if the Tea Party’s fringy elements needed any help. Before this, the dishonest critic could simply point to the scary, straw-man caricature that could be painted of the Tea Party’s most extreme elements on behalf of the rest of the movement. (Common cheap shot: “Look! They’re just racists!”) And more honest critics could cite both anecdote and poll data to question how mainstream the movement really is. (And mainstream it ain’t.)
But Levin and his band are now trying to out-distort the death panel crowd. And the right is trumpeting news of this “crash the Tea Party” effort, because it gives Tea Partyers a way to blame the ugly antics at their rallies on malicious outsiders. Americans might be more willing to believe that the clownish behavior they see at Tea Party rallies is actually the work of Manchurian candidates. Also, representatives of the movement, such as Connecticut organizer Bob MacGuffie, get to say things such as this to the Associated Press: "[Tea Party opponents] can't actually debate our message." Actually, Bob, they’re not even trying to take you on intellectually -- though that is surely doable. Which is exactly why this tactic is so despicable."
PostPartisan - Sabotaging the Tea Party