If you just play the game and have fun, you will get the weapons.
When you tie achievements to new weapons, you force people to play for the achievement rather than for the match. This encourages people to do things they otherwise wouldn't do to unlock everything as quickly as possible. Even a seemingly normal unlock becomes an artificial grind when there is a reward tied to it - the seemingly normal ones taking a backseat to the ridiculously unrealistic ones.
These are a bonus, the game does not require them to play.
That's the current meme, but it's only half true.
They are forced on people who want to be on an even footing with everyone else. Without them, you have less options than the guy next to you, and he only has them because he significantly altered his playing to unlock them. Either that or he cheated. This has created a situation where you are at a disadvantage without the options offered by the unlocked items.