You can draw similarities to many activities. "Hey, my band is going to be performing at Battle of the Bands. Come on out and watch us and cheer us on and we'll head on out afterwards to Club Che-Divebar to celebrate with drinks" or "Hey, our theatre club is performing on Thursday. Come on out and watch and we'll be hosting a reception afterwards".
If a friend in those instances didn't come to see the activity, but showed up for the after-parties, I would probably hold a negative view of them. Especially since the after-parties cost money, and are to celebrate the performance. I would view it as them wanting the free food/drink rather than actually supporting me. In the OPs case, I would probably feel the same. Especially when the bill came.