“By design” can have two meanings:
- the design is deemed to be superior to the requested design; or it what requested more often than any other design
- an issue is a consequence of a design that was done several years ago and this particular point was not under consideration at that time; the current design does not allow the desired feature
I think in this case it is the latter. I assume when Steinberg designed this area of Nuendo/Cubase they simply missed this point (there can be a dozend or more things to be considered for each new development). Maybe it cannot be changed so easily now, as it would mean to change some of the core of the software.
All of this is just an assumption, of course, but I don’t see anybody thinking that the missing undo is a good idea.