Downvotes aren't "I don't like you / your idea". Obviously some people use them that way, but ultimately they're about public moderation.
I downvoted your comment. I can explain why if you like. It's not because "I don't really care about what you're saying", and it's not right to be so dismissive. There are substantive reasons.
First, you set up a strawman: Why are Americans so obsessed with Nazis?
There are two implications here:
* The first is that there is an obsession, an irrational amount of attention. Obsession has strong negative associations. It's a highly loaded word. This is a common rhetorical pattern: choose such a loaded word and present it as the antecedent of whatever argument you're putting forth. It lays the groundwork of the ensuing discussion that, indeed, Americans are obsessed, why would anyone not agree with that? It's often used as part of a Gish Gallop, where highly editorialized statements are peppered throughout an argument, making a thorough response become far too long-winded, meandering, and rhetorically weak. It's a strong signal that someone is posting in bad faith; there are many ways to ask this question in a neutral way, but that wasn't done.
* The second is that Nazis are the issue, rather than fascism or the right-wing in general. It's definitely worth noting that this article specifically used the term "Alt-right", and only referred to Nazis when referring to actual neo-Nazis, Nazi symbolism/flags, etc. Generally, discussion of Nazis is common because it's an entirely apt and powerful example of right-wing ideology and political power.
Beyond the rhetorical flair, the rise of right-wing ideology in America has been so sudden and pervasive that it strains credulity that anyone would be surprised that the issue gets lots of attention. The rhetoric of foreign threats, heterodoxy and distrust of academics/authorities/elites, national/racial pride, traditional values, etc. are all shared between the contemporary alt-right and the Nazis, so these comparisons are bound to happen, and are often apt. Certainly it's not always used appropriately, but nothing can change the fact that these are, overall, similar right-wing movements.
Your comment went out of the way to bring up "obsession" with "Nazi hysteria" despite having nothing to do with this particular article, so it's hard to believe it's made in good faith. Perhaps it is, but the cost of false-positives in this sort of moderation strategy are very low; you can easily get around it by taking on a somewhat more neutral tone.