I'm not defending the OP's GM, nor am I saying I agree, but I understand the desire to limit knowledge checks on monsters. For one there's no clear-cut definition of exactly what info the PCs know based on their knowledge check. For another some PCs have a ridiculously high knowledge skill.
For example there is an inquisitor PC in my current game. She has Knowledge: Dungeoneering with 4 ranks. Her total check for monster lore against dungeon dwellers like aberrations or slimes is +15 from stats, a magic item and skill. Even on a 1 she gets some basic info from a CR1 or below monsters; her average of 25 (10 on a d20 +15) is so high compared to monsters of the party's APL that I usually just read most of the Bestiary entry.
If you're playing a gritty or horror-themed game or the GM is trying to keep monsters fantastic in some way such high checks can really deflate the mood quick. "You see a hideous, lamprey-mouthed humanoid with gray flesh and tentacles..." Knowledge check... "I mean, you see a Choker, CR 1 with a Grab attack and much of its defense wrapped up in high Dex. Please note the Aberration qualities here..."
Anyway, that's my 2CP.