Litebulbs, you are correctly picking him up on the use of the word legally, poor choice of words on his part. The message is however correct, in that an employer can sack however they like, just like a thief can steal from whom ever he likes, a price may need to be paid for both actions however, or not!
In the case of Mr Everard however and as someone who also has to (unfortunately) deal with a lot of disciplinary processes this would have been an easy one. The facts that we are aware of alone (there could well be other transgressions) would have certainly been enough. This chap linked both himself and his employer (and less importantly in my view the PCCC) to a web site presenting pornography. His continued employment was certainly in jeopardy from that point in.
As a Union rep yourself I’m sure that you wouldn’t consider his actions as being wise or productive to say the least. Indeed I’m surprised that a professional union hadn’t sacked him as a representative themselves long before BA were required to take action.