It's often a stupid rule. If you have a process that is stuck, you type "kill <pid>" to kill it. But kill doesn't tell you if the process was killed or not, so you have to double-check with "ps <pid>" to see if it is still alive. If it is, you try again with "kill -9 <pid>".

I suspect the reason is that for most signals, kill can't determine if the signal was acted upon or not. But for KILL and TERM it could wait a few milliseconds and then print if the process is still alive or not.

Edit: those who are saying that I can write a wrapper script is missing the point. The point of computers is to be useful to their users, not to follow some philosophy people invented almost 50 years ago. Like if someone is bothered by kill showing messages, they can write a wrapper script (kill > /dev/null how hard is that?) or beg the developers to add a -q option to kill (like grep has) or write a new tool for sending signals.

Also, the program is called KILL so one could be forgiven for assuming it's main purpose is to KILL things...