It's a library, not a cafe. Are you a toddler who has to be constantly placated with snacks?
Until recently, I too worked in a University Library. We had a simple policy - bottled water only. People still tried to eat though. The issues with people eating:-
Firstly, the mess. Grease, crumbs, wrappers, spills, finger marks, over tables, PCs, books, DVDs. Keyboards out of action after people spilled fizzy drinks. You should always leave the library as you would like to find it. If you treat it as a dining room, don't be surprised if tables are greasy, keyboards are messy
and books are ruined. Being a student is about respecting communal areas and the facilities for learning provided for you. Would you also expect to eat in the labs?
The library budget is not finite. More money spent on clearing up crap, replacing damaged items = less money on resources.
Secondly, the noise. Crunching, munching, chewing, rustling, fiddling with wrappers, noisily licking fingers, sucking. In a quiet area, EVERYONE can hear you. During exam time, your fellow students WILL complain about you and expect library staff to shut you up or evict you.
Thirdly, the smell. Offputting to those using some of the study rooms when they stink of takeaways.
Fourthly, cultural issues. In the library I worked in, many of our students were Muslim. Should they have to sit in the debris from a previous student's pork pie fest, or surrounded by half eaten Greggs sausage rolls?
Fifthly - pests. Eating food leaves crumbs and debris, and this attracts mice, especially in older buildings. Again, damage to fabric of building, wires etc.