(Details Question) What's wrong with this in a business/office sense?

I work for a university foundation program in China. My office employs about 20 teachers, and I am in charge of the “exam preparation team”.
Our function is to prepare students to take official tests that will allow them to go abroad. Every 10 weeks we conduct a practice test for these students to track English proficiency progress. Grading these practice tests is a struggle and a labor of love.
I have to call employees outside my ‘team’ to grade at times and they have to put in duty hours—that they are contracted to do, and even though it’s fair to call them, I hate taking people’s free time.

Here’s what I’m wondering: my boss has discouraged me from treating graders to an order in McDonald’s breakfast before and as they grade. I do it because it’s a thanks to them for helping me, but the boss doesn’t agree. What am I missing?
P.S. I still buy coffee…