There were too many issues. The mail archive tells us that there were considerable discussions and thoughts behind the current behavior:
http://thread.gmane.org/gmane.comp.version-control.git/49694/focus=49922
http://thread.gmane.org/gmane.comp.version-control.git/46341/focus=46818
http://thread.gmane.org/gmane.comp.version-control.git/54020/focus=54132
http://thread.gmane.org/gmane.comp.version-control.git/21949/focus=21987
The last message mentions a bug in "git rebase" that internally uses "git am" when replaying the changes. It was corrected later with:
http://thread.gmane.org/gmane.linux.ports.sh.devel/3833/focus=79735
Reading the discussion in the messages in this thread will help understanding the issues, after understanding why "the first line" was problematic, as discussed in the other threads.