The FA deemed the term to be a racial reference, but Ferguson believes that Ferdinand's status in the game, and the fact that he plays for United, had something to do with his punishment.
"It was almost certain to happen. [The FA] want to close the door on this kind of thing. What surprises me is that other players have been twittering for ages and have never been challenged by the FA," said Ferguson.
"His status in the game has caused that and who he plays for is maybe another issue.
"Twitter; I do not understand it. I don't know why anybody can be bothered with it, but it is there and as a club we have given instructions to the players that they cannot talk about Manchester United. Rio is mature but he still has to remember that one word can make a difference - and you can't take it back."
Manchester United's first match of the new Premier League season comes away to Everton on Monday.