Let's review what 4E is all about:
PHB, page 6: A Roleplaying Game
While the D&D game uses dice and miniatures, the action takes place in your imagination. There, you have the freedom to create anything you can imagine, with an unlimited special effects budget and the technology to make anything happen.
The DM makes D&D infinitely flexible - he or she can react to any situation, any twist or turn suggested by the players, to make a D&D adventure vibrant, exciting, and unexpected.
The DM sets the scene, but no one knows what's going to happen until the characters so domething - and then anything can happen!
In an adventure, you can attempt anything you can think of.
PHB, page 286: Actions in Combat
This section describes how to perform the most common actions that are available to you on your turn.
The list isn't exhaustive - you can try to do anything you can imagine your character doing in the game world. The rules in this section cover the most common actions, and they can serve as a guide for figuring out what happens when you try something not in the rules.
DMG, pag 42: Additional Rules - Actions the rules don't cover
Your presence as the Dungeon Master is what makes D&D such a great game. You make it possible for the players to try anything they can imagine. That means it's your job to resolve unusual actions when the players try them.
There are a lot of ways to resolve disarms. The DM has to resolve it appropriately for the group he's in. But to say that disarms are somehow not a part of D&D is a really strange statement.