The law that describes this is the National Security Act. The relevant passage from the wiki states (emphasis mine):
In other words, it made communism illegal; recognition of North Korea
as a political entity; organizations advocating the overthrow of the
government; the printing, distributing, and ownership of
"anti-government" material; and any failure to report such violations
by others illegal. It has been reformed and strengthened over the past
few decades, with the Anti-communism Law being merged with it during
As for application of the law, there is another passage:
The South Korean High Court has a ruling history since 1978 that has
classified 1,220 books and print material as "Enemy's Expressions" by
force of precedence. Two state-established research institutes decide
what books and print materials meet the criteria of "Enemy's
Expressions": the Democratic Ideology Institute, established in 1997
under the direct orders of the Chief Prosecutor, and the Public Safety
Affairs Institute of the Korea National Police University.
However, since the early 1990s, the Public Prosecutor's Office has
chosen not to bring any citizens (or publishers) to the courts for
what's deemed by common sense as not risky. Courts still invoke
the law when increasing fines or years in prison for political charges
against what the South Korean state deems subversive- in most of the
cases pro-North Korea- groups.
So in short, I think that as a tourist you should be fine to visit no matter how left you are. Just don't shout that Communism is the best and North Korea is the only Korea in front of a police station and you should be fine.