Because free speech is an essentially British tradition and we should stand up for British traditions such as this.
What it says:
A philosophy don came to speak at my school when I was much younger. He mentioned in passing that the economic assumption that people always act selfishly was clearly not always valid.
For example, he said, "if that were true we would not get such a thing as a suicide bomber".
We can argue about whether this is a valid line of argument but people should certainly be free to say such things. Keep the offence of incitement if you must, but let's get rid of this silly "glorification" nonsense.