This honestly surprised me quite a bit, but to be completely honest I think it's a cool design at the end of the day. The tanking spec design just works. Feral druids suffers the same problem, except they just overbloated our tree instead.
And to the people are sad they are killing your fun: It has been announced that for Cataclysm tanking class offspecs will be able to tank heroics with a bit of thought. That means Arms and Fury warriors can tank your heroics, and unholy/frost DK's will no doubt be able to do the same.
So don't worry too much. You'll still be able to log on your alt and frost-tank!