Someone posted an Adblock link on how to remove it in the locked post if it's that annoying to you.
Honestly, I did it, just because I don't have the money to donate, and my OCD was really starting to kill me having the giant notice across the top of the screen.
I still don't understand why we can't cancel it, but meh. The only response from the admin was so that new people who didn't see it would get to see it... but if we dismiss it, new people coming on would still get it themselves. Heck, I came back after ~6-8 months of being gone, and I had a very very large list of notices.
However, natural disasters that cause this much damage are posted on many sites. Not just Pokecommunity. Gaiaonline, Facebook, Many Social Forums, etc. have similar donate banners/threads. The same was with Haiti, and Sumatra back in 2005. It's not just because Japan is where Pokemon comes from. Thousands of people died. Hundreds of Thousands are without houses.