Hi,
I really think it's a cultural issue. In some societies it's probably one of the rudest things a student can do. On the other hand there are some countries where it is probably normal conduct and completely acceptable.
Another thing to think about is that there isn't always an organized lunch break and therefore students find themselves starving in the middle of a lecture. I think that losing concentration, thanks to hunger, isn't the most desirable thing.
SmithDerik's point regarding food being a concentration booster is also a good one.
Have a nice day.