I know a couple people (including myself) were a little upset that the SD version wasn't just updated to be Universal... but now the HD version is free... so we can all play on our iPads in normal resolution. Yippie.
Far be it from me to begrudge free-ness, but why only the iPad version?
I'm just guessing but possibly because there were grumblings among folks who bought the iPhone version assuming it would turn universal so they could play it on their iPad. This gives them the chance to pick it up for free instead of having to buy it again.