I did not click YDI, but because I take buses to an from work every day, I can try to guess why someone might. OP is British, so by "chips" he likely means what US folks would call "fries," not "potato chips." Eating on buses is rude enough by itself, but eating foods (especially hot foods) with strong aromas that everyone else on the bus can smell is even ruder. Especially to people who are hungry or on a diet or allergic to whatever food they're being forced to smell. Similar to sitting on a bus near someone who is wearing way too much perfume. Conversely, if it really is potato chips being discussed, then we're potentially talking about lots and lots of noise (crunching, and possibly other more disgusting sounds if OP does not chew quietly). But regardless of whether either of these things is true, spitting in someone's food is a totally douche-y thing to do, so I don't think it could be "deserved" under any circumstances. Never answer rudeness with more rudeness. It only doubles the rudeness.