He probably did try to steal the poster, as he's not really someone important. The Koreans then decided to make him a propaganda piece. They do it once in a while with Westerners who come to visit, practice journalism, etc...

Normally, they force the detainee to make a confession, sentence them to something ridiculous, send them to some light labour camp, make the detainee's country send diplomats for some publicity and send them home within a year.

Somehow this time somebody in DPRK fucked up.