I saw Kelly Clarkson and Adam Levine do this during the Honda Civic tour, it was much more noticeable. I've also seen Elton John using this method too during his Red Piano Show
On one hand, I can see why this is kind of sad and shameful of her to do, but on the other hand Gaga was photographed with the lyrics to "Gimme Shelter" with the Rolling Stones couple years back. Personally, I feel like if it's YOUR OWN lyrics to YOUR OWN song that you supposedly wrote yourself, then I think you should have them memorized, but I guess sometimes in the moment, I can be a little straining, cuz I'm sure Katy's not the only one to have done this.
so what, is she expected to memorize everything all the time and not take advantage of this great technological aid? if anything, it proves she's not lip syncing and that she was actually singing! :-D