The debate is not over totalitarian regimes but over symbols, flags and emblems. If you don't understand the difference between what the communist flag and the Nazi flag represent, you may find it hard to understand why Nazi symbols are banned or restricted in various countries [1], while communist/socialist flags are waved with pride to this day, not only in many Western democracies but even in Russia [2]. In my country (Israel, where over 10% of the population is comprised of people who fled Soviet oppression), communism/socialism has been practiced (although not on a national level) for over 100 years in a democratic way, and on May 1st, kids in leftist youth movements carry red flags and sing the Internationale.
Communist symbols (like, say, the Spanish flag), despite having been adopted by some of the most horrific and oppressive regimes, do not represent oppression or dehumanization, and they have also served as the flags of peaceful, democratic parties and organizations. The same is true for capitalist symbols, which, despite genocidal regimes (e.g. Belgium) are not recognized as hate symbols. Nazi symbols, however, are qualitatively different.
BTW, to the best of my knowledge, even the professor in question is not posting pictures of Stalin or the Khmer Rouge flag.
[2]: Here you can find a map of countries with at least one political party that's a member of the Socialist International, where communist/socialist symbols/emblems/flags are celebrated https://en.wikipedia.org/wiki/Socialist_International
Of course I'm aware of the purportedly benign displays of symbolism by social democratic organisations, and I don't have strong objections to those.
But I'm also well aware of the violent instincts of some people on the far left (including people I know personally), who invoke the supposedly well-intentioned philosophies and symbols of communism to legitimise dark fantasies about violent attacks on others based on their race or class. I don't know much about the professor under discussion here, but from what I can see he doesn't seem to be doing much to distance himself from such conduct, and shows some signs of revelling in it.
As someone who was raised on strong social democratic values, I now often find myself shocked at how much identity-based bigotry and rhetorical violence is tolerated or endorsed by people who claim to be of the left and who claim to stand against bigotry and violence.
But most importantly, I really can't tell you how much I hate to be having this discussion on HackerNews. It is simply not what this site is for, and as a prominent member of this community, I really wish you would think better than to engage in this kind of campaigning here.
It does nothing but make this place less pleasant and less interesting.
So much on this site is political -- as is much of what is produced by the tech community -- and this discussion, which I did not initiate, is an example. Unfortunately, much of the extremely political debates on this site and in tech in general is done under a veneer of the avoidance of politics. Politics, which is the process by which power (influence) and resources in society are distributed is something that cannot generally be avoided, when the topics discussed are those that appear so frequently on this site. The only difference between politics done under the cover of the apolitical and an overt political discussion, is that the former tends to be a particularly bad and harmful form of politics (as the actual topic of discussion, the distribution of power and resources, namely politics becomes implicit and hidden). As this site is already extremely political -- it is certainly the most political among technical forums for tech professionals -- the least I can do is make the pervasive politics overt. People who frequent this site (as opposed to say, /r/programming) are already exposed to hefty dose of politics, and those who dislike politics probably do not visit HN, and if they do, they avoid items such as this one.
As to the rest of your comment, I don't see what the behavior of some has to do with the meaning of symbols. There are plenty of horrible Americans, including American leaders, who use patriotism and Americanism as a justification for terrible actions. That doesn't mean that Twitter should designate the American flag a hate symbol. Communist symbols fall in the same category, while Nazi symbols do not.
Communist symbols (like, say, the Spanish flag), despite having been adopted by some of the most horrific and oppressive regimes, do not represent oppression or dehumanization, and they have also served as the flags of peaceful, democratic parties and organizations. The same is true for capitalist symbols, which, despite genocidal regimes (e.g. Belgium) are not recognized as hate symbols. Nazi symbols, however, are qualitatively different.
BTW, to the best of my knowledge, even the professor in question is not posting pictures of Stalin or the Khmer Rouge flag.
[1]: https://en.wikipedia.org/wiki/Post%E2%80%93World_War_II_lega...
[2]: Here you can find a map of countries with at least one political party that's a member of the Socialist International, where communist/socialist symbols/emblems/flags are celebrated https://en.wikipedia.org/wiki/Socialist_International