I think that the club have been a little weak in just accepting the punishment if I'm honest. I think that they should have made a real fuss about it and carried on fighting. There was contact and Niasse went down. Whilst he may have gone down easily, Dann's intention was to impede the player, which is a foul.
I Don't understand the idea that if It's anywhere else on the pitch, It's a foul but if It's in the box, It's soft. A foul is a foul. Niasse wasn't screaming for the penalty to be awarded.
I think at the very least, the club should have questioned the ruling, based upon numerous incidents already this season that have gone unpunished, and put real pressure on the FA to explain why. It may not change the decision, but it at least plants the seed that the FA are corrupt as fuck and bend over backwards to ensure that the top 6 get away with fucking murder.