That's a terrible thing to be considered normal. Those students were on break, on vacation. I didn't do work for my job when I was on vacation!
I hate cheating, but I hate that you made them take an exam on break even more.
Edit: "Class, I see some of you did not understand the way I require work to be shown. For this reason, I will reteach my requirements. Those of you who did not understand will be given an opportunity to retake the exam."