While I think what happened is fair, we don't do this at the WSOP (unless there is a gross misunderstanding) and I agree with the principle of why not.
Despite all the people involved in this pot that can't count, there is one constant and that is that Player B is all-in. Player A knows that Player B is all-in. This means that if Player A says call, they are "accepting the action". In this situation a variance of 2,000 in chips is not a "gross misunderstanding". Player A knows he is at least facing 7,400 and if they were unsure they should have asked for an exact count.
A gross misunderstanding would be if Player B had 74,000 in chips and Player A thought they were facing 7,400.
As JasperToo pointed out because verbal takes precedence, even a gross misunderstanding of 74,000 in chips would be challenged as Player A stated verbally "CALL" as verbal bets are binding. Period.
I once had a ruling where the blinds were 300-600 and the standard bet was 1,800. After betting 1,800 many times in a row, Player A accidentally said 18,000 and put out 1,800 in chips. Player B immediately said call and started to put in his whole stack. Player A says, "I thought he said call?" and I had to tell him that he said "18,000" and that Player B was calling 18,000 (though Player B was actually all-in for 17,000).
It's just another example of us floormen trying to decipher what is a lot of the gray areas in poker. What's best for the game. When to follow the letter of the law. But that's what we get paid for. Each example is a case by case, just like any Judge. Some people get let off the hook, some people get 10-20, some people get tournament life!