NO rationale? maybe it used to... Using spaces between words in Japanese elementary school textbooks

This note is continued from the previous one.
Recently, I've got several chances to know how textbook company insert spaces for Japanese elementary school textbooks.
This is a kind of mystery since I cannot any rationales in anywhere!!

After contacting several company people, I've got why I could not find any clear answer.
I found that this does not come from my poor research power but from historical problem.

History was like this, (I want to say "thank you" to Tokyo-Shoseki and Ishidoya-san at Sanseido)
At a certain point of time (1975), Education department of Japanese government issued
how to make textbooks for elementary school.
In that book, there was a rather ambiguouse phrases about using spaces for the first two year books.
it said that textbook company should use spaces between phrases for first two years because
younger children did not know much kanji and, then, they were hard to know the boundary of words from kana sequences.
But, at this point, there were no clear criteria for how to divide phrases.
And, things were going worse, in revised policy statement, description of using spaces were deleted.
So, now, there is no clear rationale of using spaces in elementary school textbooks in terms of government policy.
and, of course, rationale was absent from the point of scientific view, either...