...Every time Hitler's name is mentioned people freak out, sometimes rightfully so. But in this case it was appropriately used.
It was appropriately used - Charles was comparing Putin's actions in the Crimea to Hitler's in the Sudetenland.
I think there are a few problems with the comment, or at least with the reaction.
First there's the fact that it was made to a woman who is related to holocaust victims. When you have a headline of "Prince Charles compares Putin to Hitler to the Family of Holocaust Victims" the go to thought isn't that he's making a comment about foreign invasions.
Secondly, comparing people to Hitler - however valid such a comparison might be - is such a loaded statement that there is no way things can go over well. There is a concept called Godwin's Law which states that the longer an online discussion goes on the more inevitable it is that someone or something will be compared to Hitler or Nazism. A corollary is that as soon as Hitler is mentioned the debate is over, and whoever mentioned Hitler has lost. Comparing people to Hitler is often counterproductive in that sense.
Thirdly, Charles is a man who is in line to be an apolitical head of state. For him to make ANY public comment about a foreign nation, its head of state, its policies, etc, when he's not speaking on behalf of the government(s) that he represents is hugely inappropriate. This isn't simply a blunder for the British, it's also a blunder for Canada. A member of the Canadian Royal family, the future head of state of Canada, while on an official engagement in Canada representing the current head of state went and made a comment comparing the head of state of another country (one that is actually fairly close to Canada, militarily better off than Canada, and has had a tense relationship with Canada in regards to Arctic sovereignty) to a man who is often considered to be the worst human being ever.
While I don't disagree with the comment and think it is an apt comparison, I do disagree with Charles' decision to make such a comment in public as saying such things is kind of contrary to his role.