What triggers them is any suggestion that the Nazis were good and suggesting the surrender of St Petersburg to the Nazis would have been great definitely falls into that category for the majority of Russians.
On top of that St Petersburg really has a problem with violent Neo Nazis groups, or at least had - don't know how it's today.
I'm actually pro freedom of speech to a degree that most people in Europe are not (I would allow them have their stupid demonstrations) but the truth is that in most countries some ideologies are forbidden. Nazism is forbidden in most parts of Europe or how about showing public support for ISIS.
I'm 100% sure it is forbidden everywhere in Europe (maybe with UK being the only exception).