I agree that managers should be able to comment on dubious decisions without fear of punishment. However, there is a difference betwween passing a comment/observation and simply being abusive and offensive. Unfortunately many managers are very offensive in their comments and for that they deserve punishment.
And in my view, any player who is abusive to a referee or linesman should be dismissed instantly. Football really should follow the example of rugby in this area, where there is zero tolerance of such behaviour.