Why? This is a new one on me.....
The post that mentioned the capture of the M&S sandwiches was moderated into oblivion (no complaints; it was a blatant drift), and mentioned that the security staff may have been feeling peckish.
But raffele's explanation may be better; mayo is a dangerous explosive in the wrong hands.
If you are worrying about the mayo served airside, fear not. It has all been carefully inspected, sampled and tested. Just like every bottle of liquid sold to replace the confiscated items, at a much higher price.
What's that? Did you say a rude word? No, honest, it has.
Hasn't it?