This is no doubt due to Peter Capaldi's appearance in the episode. They also made Fear Her free around the time of the 2012 London Olympics.
I went back and rewatched Fires of Pompeii the other night. I'm sure now that some parts of fandom (the fanwanky, annoying parts) are going to want to retcon the 12th Doctor somehow in that story and explain why he was there and didn't tell ten who he was. I really, really hope that there is no mention of that story once 12 appears. More to the point though, I had forgotten what a powerful story Fires of Pompeii was... they only bits which were a let down for me were when Donna and the Doctor were running around inside the Volcano which stretched credibility a bit. I think it definitely showed what kind of companion Tate was going to be after some of the silliness of "Partners in Crime", and hopefully anyone who didn't like her because of her comedy quickly forgot that fact, she was truly brilliant, in my opinion at least. Meanwhile I'd also like to go back and see Capaldi in "Children of Earth", but to be honest that whole story freaked me out so much I'm not sure I could watch it again. Which shows how good it really was.
If you did try to retcon Capaldi's appearance as significant, you're going to be busy, what with two companions also hanging around there too (as well as Karen Gillan, Capaldi's 'wife' Tracey Childs plays the 7th Doctor's alternate universe Nazi scientist companion Klein in the Big Finish CDs).