I wouldn't say that its intended, I could see the point if it actually did damage but the way it is its just weird if you can't hit something with a diamond swort but you can with a sharpness 1 wooden swort.
I think it should either be, allways beeing able to knock mobs back or only knock them back when you actually do damage to them, this way its just not consistent.
Reproduced in the vanilla client after doing a forceupdate.
The gameclient is vanilla, therefore it is a vanilla minecraft bug.
Adventuremaps use custom potions (which are a feature of the game, specificly added to be used in the way I did) therefore this should be fixed and not ignored.
Tested and confirmed in 1.5.1