Is volunteering illegal in Korea?

This is my third post this evening, but it is the one I find to be actually important for foreigners here.

Some foriegners in Busan put on a play or comedy performance and were busted for not having the right visa or permission for it. I'm neutral on the subject but while being questioned by the police, they were told volunteering without first asking for permision from a customs office was also not allowed.

Volunteering is a fairly important part of my life. I guess I shouldn't say whether I am currently volunteering in the current atmosphere (although I did a few posts -back in those happier times).

The Korean blogosphere is active on the subject with the best post on the subject coming from Nathan, the Seoul Hero.

Nathan also comments in a separate post about students asking for better grades. I guess it is common to be able to do so in Korea as my university has the same policy. Now, I do make mistakes so I don't mind the ocassional student asking me to double check my math but there are also students who want to suddenly make up for a semester of slacking. Twice now, once this year and once a few years ago, I had students offer to do a new assignment over the winter holiday if only I would increase their grade. This semester, four students have asked me to change their grades. For two of them, I am sympathetic and will see if there is anything I can do; they were hard workers in class and I am happy to return to their exams and see if I find a mistake on my part for them.
For the other, I will make the effort but with less sympathy. In one such case, the student's father called me and asked if I could make changes. That student is in for a rough winter break if I can't but that isn't my problem.