Yes, it's very frustating, I experience the same thing. I also can't change the names on the one Calendar Group I was able to set up. Everytime I type the label "Personal" instead of "the standard lable "group" the Mac pings at me and changes the label back to "group"!!.
There is no info on this topic in the iCal help menu either!! C'mon Apple, there is something up with this feature!!