But that is dissent which he got found not guilty of.I think it's more about how he you appeal and the reaction if the decision does not go in your favor.
He got found guilty of over appealing because he appealed without ever looking back at the umpire (which as mentioned other bowlers do also). Now to me that isn't as bad as some bowlers constantly appealing when they (and everyone else) know the batsman isn't out.
Anyway, I do think Mills was worthy of being fined, I just think others need to cop a bit of punishment for "over appealing" also.