I wish I could find the source, but I remember reading the floor of the hotel that Warmbier took a poster from was a floor that was supposed to be sealed off because it was used by the military - essentially like a military base. AND that he was specifically told not to go there beforehand. So basically you have an American just waltzing around a DPRK military base after being told not to. So he got 15 years and it was completely reasonable - and not because he “took a poster”. Hell, if a DPRK citizen snuck into Whiteman AFB, they would rot for decades in a max security prison (at best).
The fifth floor of the hotel is sealed off (the elevator doesn't stop there), although we're not sure why exactly. At his trial however, Otto said he took the poster from an employee-only area on his own floor.
The charges brought against him were of collusion with a foreign state (the US), he was sentenced for subversion. The video of the trial should be online as many western media sources used the footage in their own reporting, but I haven't been able to find it. In his confession and closing statement, he said that his church (Methodists, if that makes a difference) and fraternity wanted a trophy from his trip.