TKD wrote:

This is because season ticket holders using prepay as extensions do not
see
themselves as "prepay users". The signage tends to say "prepay users must
touch here" but you have to realise you are a prepay user despite having a
season ticket if you are going beyond the zones on your travelcard.

And of course yet another point where the whole "no need to plan in
advance/change your destination as you travel" falls apart!