The Evil GM - Meta Gaming Pisses me off.

Today I was cruising around on the Wizards of the Coast forums, and looking for some 5E stuff and I came across this post, entitled âKnowYour Enemyâ.
I figured, "hey this could be interesting" as I opened it up awaiting the slow speed here at work. My eyes narrowed and then became wide as I noticed the last line of this post:
ââŚThis guide will attempt to do extensive analysis of the monsters, as published, so that you may better make informed decisions and avoid any unpleasantnessâŚâ
So I continued to read this, and the guy really went into detail of each of the monsters even providing charts.
This was excellent PLAYER knowledge, NOT for CHARACTERknowledge, but itâs written for players who want to Meta-Game think. This in my opinion, defeats the whole purpose of playing the character in the game. Why? Because how would a level 1 character whoâs never experienced anything know to use this or that spell, or weapon on a creature they've never seen?
OH I KNOW HOW, META GAMING.
This stuff pisses me the f#$k off and is also the main reason, I change every creatureâs stats, increasing (and decreasing) AC. No creature in my game is a âStandard stock from the bookâ creature.
Can I do this? Yup, I am the DM, I am running the game, and itâs my world.
I understand if youâre running a module and itâs already printed there, and you want to keep to the notes to make your life easier, but you can still change that as well.
I am not sure how people can play a game and enjoy doing this.