Perhaps "I'd agree" was a little strong. What I meant was "it wouldn't be that aweful except that self defence isn't a 'good reason'".
If self-defence was a good reason, then the only people who could ever be convicted would be would-be-murderers etc (remember, the police must prove that you meant someone harm).
Now, suppose we can't prove who is a would-be-murderer and who isn't, well then no one gets caught by the law as you are innocent until proven guilty - it would be a pointless law, but no innocent people would be punished by it.
Unfortunately, as it is, self defence is not a 'good reason', so the law sucks.