Well it falls under unfair play because both batsmen can't anticipate the pause and how long you'll pause for. I personally don't mind a pause either, but when you leave it open to umpire interpretation and have grey areas, it can lead to trouble.
For example - say you're bowling as usual, and the non-striker times his running based on when you release the ball under your regular action. One ball you suddenly pause in your action, and then catch him out with a mankad. That's unfair isn't it? He wasn't backing out of his crease unfairly trying to take an advantage, he was just running based on your regular action. He was doing a perfectly legal thing till you deceived him with the pause. Getting a dismissal this way goes against the intention of said dismissal. Therefore, there needed to be a rule to ensure that, in such circumstances, the non-striker wouldn't be out.
Similarly, how long is a pause justified for? At what point have you gone from 'pausing' in the action to then simply not releasing the ball? A batsman will understandably lower his guard when he see's you've stopped in your action - if 5 seconds later you whirl your arms around and fire the ball into the stumps, is he to blame for not being ready to play it? Do we leave it in the umpire's hand to determine when the pause has gone on for too long? You know they'll be inconsistent with their application of it. They are only humans. Or do we add to the list of tasks the umpire already has and ask him to start timing how long the pause is, and introduce an arbitrary cut off in the laws now as to when a ball goes from live to dead during a pause in action?