Well, the blizzard keyboard API specifically allows this.
And addons such as Snowfall and Macaroon take advantage of this functionality.

It's your call to use it or to not use it.
But Blizzard specifically put this functionality into the game, it only works because of their Keyboard API, and will stop working (no matter what the software tries to do) if they ever change their minds and change the Keyboard API functionality.

The one key = one action quote is frequently taken out of the context it was originally given. It refers to you do one action on your computer and one action occurs in the game. That single action can be multiple keys without any issue. It specifically refers to your not being able to push a single key now, and have something happen now, something else happen in 2 seconds and then 6 seconds later a third thing happens, all based off of the first press and without any additional input.

If you're not comfortable taking advantage of a feature which Blizzard chose to implement in their game, then don't use it. It is the same as not using Interact With Target and Click to Move, which honestly were put into the game to help handicapped players with one arms and such to play the game, not to allow melee boxing to swarm players as they move about the battlefield.