That's a very poor assumption. But even if it weren't, it would, on its own, be insufficient to justify the practice.
"The reader is assumed to be an intelligent rational agent. That's a very poor assumption. But even if it weren't, it would, on its own, be insufficient to justify the practice."
It's not a poor assumption. It's a good assuption. In fact, prior training in the matter is necessary in order to read and comprehend the material. the practice is entirely justified, because to do otherwise would over complicate any work with an extreme amount of redundant, unwarranted comment. Where appropriate, footnotes are given.