Can't beat the assholes join the assholes. If i'm not mistaken post-grunge is a subgenre. You see anywhere on the front page that asks for a subgenre? Didn't think so. It asks for a genre, and it is alternative rock. Furthermore, they do NOT, and I repeat, do NOT like to consider themselves anything related to the word "grunge".  Dave has come out before and noted that he personally "hates that word", and it even says so on their official website. Even more, if you were to even keep post-grunge on there (which you shouldn't) then there is absolutely no reason whatsoever to not include piano rock on there either. Here's the link to the website and I will try to find more regarding the bands distaste for the subgenre: 

 http://209.85.173.104/search?q=cache:X7GMQ7shxikJ:www.foofighters.com/dictionary+grohl+grunge+i+hate+that+word&hl;=en&ct;=clnk&cd;=5&gl;=us