But I have a question: the question tags are separated from the question
structure... is that to avoid to have to retrieve all the questions?
(maybe I'm not so clear)
You mean: the tag labels are separated from the question structure?
Well, the main idea is to reduce the amount of information associated
with each question. Each tag has both an integer id and a string label
and just the integer id is used with the question, so that needed
storage is reduced. Moreover, a computer is much more efficient at
handling integers than strings.
Last but not the least, as it is expected that tags does not evolve that
much, they can be retrieved from the demexp server and stored somewhere
to be used more conveniently.
Does it answer your question?