Why aren't official symbols of vanquished enemy militaries a violation of the surrender treaty?

Surely we all know that the whole reason that fringe groups use symbols such as these is to garner the kind of attention they crave. Attention allows them a platform to get their reprehensible views out in the public, and further an agenda.
Supremicist groups have been withering and dying for years, and continue to do so. Recent events were actually a sequel to a similar one they did back in May, but didn't get the attention they wanted. This time they decided to put out where and when they would be holding their rally on student websites at the nearby University of Virginia, with predictable results. Counter-protesters show up, spoiling for a fight, and they get it. Viola! News coverage and interviews follow.
Don't give them attention, and they go back to their holes in the backwoods and post conspiracy theories with their handful of like-minded friends.
The sonsofbitches have a right to speak, but I have an equal right to not listen. Or speak against.
As for killing Nazis with a smile on my face, that never goes out of style.

Because they are history and when you attempt to squash, erase and remove what they meant and the reasons why they came to power you fail to learn from history. What happened to Imperial Germany after world war I lead to the rise of Nazi Germany. Winning isn't all there is you have to bring those who lost into the discussion and not grind them into the ground or you create a future enemy.

Contrary to what some may say, it has little-to-nothing to do with "protecting history". Symbols associated with a beaten opponent are often left in place for the same reason that every senior officer in the Confederacy wasn't charged with treason. Ultimately, cultural symbols are left intact as a way to help pacify the defeated population. Although many southern politicians and military leaders did levy war against the United States while still being citizens thereof (and thus fulfilled the Constitutional requirement for treason), it was reasoned that allowing them to simply fade into background civilian life would make it easier for the South to accept their defeat. To an extant, it worked. The same logic was used when allowing Japan to keep their Emperor. We could have removed him from the equation altogether, but the loss of cultural identity tied up in the royal family would have made the US occupation of Japan much more difficult.

The comparison with Germany and Nazi symbology others have made isn't entirely accurate here. Germans themselves elected to get rid of the swastikas, eagles, and other remnants of Hitler's regime. That's not a case of a loser being made to get rid of their symbols, but of a culture realizing that the symbol is too representative of an evil to be allowed to exist, even if it is part of their history.