MIG wrote:

Last week I saw notices by those readers which suggested that they're
for
Oyster users going out of zone, not people who've come down in the lift.

Yeah, it's possible that you'd enter the system by a cross-platform
interchange and/or ungated station, not needing to touch your Oyster
because of using a travelcard, and then realise that you needed to
travel beyond your zones on LU.

BUT in that case, why only at Bank and only on the DLR?

There aren't actually that many other in station Underground-DLR
interchanges that I can think of - Stratford and Canning Town are the only
ones that spring to mind and I don't know if the latter has any internal
readers, but it did used to have a non-Oyster service.

Was the original idea perhaps that DLR would be keep distinct and people
expected to always touch in upon entry.

Why can't
they provide them for that purpose on all LU platforms so that you
don't have to go all the way to the exit at the point when you realise
that you are going to use PAYG to complete an unplanned journey (or
extension of a paper travelcard as it happens)?

Because that would be too convenient for customers?