Helder Geovane Gomes de Lima wrote:

When we go to links such as: http://pt.wikibooks.org/w/index.php?action=edit&preload=Project:About&am... http://pt.wikibooks.org/w/index.php?action=edit&preload=Project:About&am... we note that both "test"and "MediaWiki:test" doesn't exist. But using the preload parameter for the first page we get the text "#REDIRECT [[wikibooks:Sobre]]" preloaded although for the second we don't.

Is this the expected behavior for MediaWiki namespace?

No, it's probably a bug. I suspect (without so much as glancing at the code) that the problem is in the part that, in effect, automatically preloads nonexistent MediaWiki: pages with the corresponding default system message when you try to edit them.