If it was Walking on Air that'd be such a dick move by Katy.
"Vote your favourite as my second single! Dark Horse or Walking on Air!

"
"Oh, BTW, Dark horse ft. Juicy J is NOT my 2nd single, it is merely a preview."
"Second single is Walking on Air, hitting iTunes on Monday!!"
The unplayed song is actually a pretty good guess, that would be a great reason not to have played it.