Yea some people do this in the workplace. I don't get it either. All you can do is politely decline to be involved and stand firm. So what if they act like you are a killjoy? That's on them.

As far as them not doing work to shop that's not ok. But unless you are in a position of authority them. You don't have a lot of options. You can tell them that you feel work comes first and it's not right to leave you hanging with their unfinished work. Work first then shop. Or maybe discuss this issue with a supervisor.