I was actually suprised at this too. I saw them on Dec 1st and I wasn't at the front of the crowd, but the doors opened at 7pm, I got in by about 20 mins past and there was only 3 rows of people, since all the people in front of me where there for Slipknot, so when they got in, they all stood around the bar and getting shirts etc, but Bodom came on within like 10mins and I saw the whole set, BUT if I had gone to get something to eat before queuing up at the venue AND put my jacket away in the cloakroom, I would have missed them completely, and like you, I only went for Bodom, so I would have been PISSED.
I read about this on cobhc.com too, where they started and the room was only getting full during HCDR, which was the second to last song because the venue let everyone in too late. It's bullshit.