convention travel documents through UK airport

[retroray]

I was admitted into France through the EU arrival gates without stamping my convention travel document (after the officer has double checked with their supervisor). which is kind of what I've expected since the TD is issued by EU country (i.e., UK).
and since there is the visa waiver agreement a stamp was not needed as a stamp means that there is immigration control which contradict the agreement.
However, on my way back to the UK, I was asked to join the none EU passport holders, to fill in a landing card, and my CTD has been stamped.
Is this the norm procedure for UK convention travel document holders when re-entering the UK? and would not the stamp be in breach of the declaration on the TD that the holder need no visa (thus under no immigration control) for the duration of the TD.
I do not mind the procedure at all, but im just curious to find out whether -in principle- the border staff knew what they were doing and whether it is the law.