TYPO3 Core

In my opinion the page should inherit the backend layout of pages of higher levels. Setting for "backend_layout_next_level" should only be taken into account for pages below.

Exactly!

Just to clarify the intended behaviour:

1. The field "backend_layout" is not inherited at all but only marks the specific backend layout of a certain page.
2. The field "backend_layout_next_level" is inherited from parent pages.
3. Any value > 0 for "backend_layout_next_level" will override possible inherited values of the parent page so pages below will behave differently.
4. Any value > 0 for "backend_layout" will override the inherited backend_layout_next_level of a parent page, but only for that particular page.

So cattura.png shows the intended behaviour, except 4), which is not in the picture.

Is it reasonable to not inheritate "backend_layout"?
Therefore setting "backend_layout" at the root page would have no effect for all following pages.

Yes it is, since this way makes sure you can override inherited values for single pages.
If backend_layout itself would be inherited too, you would have to repeat the actually inherited backend_layout_next_level setting even when it did not change at all.

So if you want all pages to have the same backend layout, you have to set both fields of the root page.
Ohterwise subpages will get the default layout.

OK.

But for TYPO3 6.2.17 page 3 does not behave as expected.

Result is "C" instead of "B".

Checked that with 6.2.17 and for me it seems to work.
Could you make a table showing the relevant part of your page tree and the settings for both fields as well as the result?

This is my configuration (backend_layout / backend_layout_next_level) - TYPO3 6.2.17

homepage (layout_1 / layout_1) -> layout_1 (OK)
  level_1_page (<not_set> / <not_set>) -> layout_1 (OK)
    level_2_page (<not_set> / layout_2) -> layout_2 (should be layout_1)
      level_3_page (<not_set> / <not_set>) -> layout_2 (OK)