should lecturers pay

I'm a PA student and because my class and the year below is small we did a get together where we ordered pizza. The plan was to split the bill between everyone who ate the pizza. The problem is that three of the lecturers came and ate a few slices and there is a debate whether they should be asked to pay.
I personally think its awkward to ask them but people are arguing that why do they get to eat it for free?

I have the same feeling as yours.
What if they were not lecturers but classmates? Suppose you have a big class and some of your classmates were not aware of the event. They passed by and came in for a casual chat. To me it's quite natural they picked some and ate. I think unless they stayed until the end, they should not be asked to share the bill.

Quite often when staff attend gatherings there is food laid on. Unless they were told specifically that a contribution was required then I don’t think they would have given it a second thought.
So it boils down to whether anyone told them.