It seemed a perfectly fair letter to me. The guy's principle complaint is against the CAA for granting the alleviation to the airline. Some years ago I took part in a fatigue study carried out by DERA (now Qinetiq) into the effects of early starts. Pilots were never allowed to see the report (probably far too contentious) but the received wisdom was that performance dropped off half way through the third early. Now, it seems the CAA have yielded to the commercial pressure from the airline.
Remember, all the airline needs is a chit from the CAA saying they can roster this and that. Then, even with a view from the office of a smoking hull off the end of the runway, they can say they did the right thing if the paper trail looks right.
So, if fatigue is cited as a cause of the accident, then the blame goes back to the CAA. If the CAA has given the alleviation based on scientific evidence from Qinietiq then they can cite that and keep their car park passes etc.
Now . . . Whilst I was taking part in the fatigue trial with DERA I had a couple of long conversations (anonymously) with one of the scientists. I didn't get any sense that they thought five early starts was okay. As I recall they recommended a limit of three early starts.
So, the letter in flight is entirely correct in my view. The writer points the finger at the CAA. The question is: why did they give the airline the alleviation in the face of the evidence?