That interview question is not weird. It is a very good question to test your legal aptitude. When you gave a 'common sense' answer you probably were thinking about how sensible it would be for the police and the courts to waste their time on trivials matters involving bags of crisps, etc. But this is not where a lawyer would begin. A lawyer would begin by trying to define a picnic. Did you offer a definition? If so, what was it? And was it precise enough to decide whether the other incidents decsribed were, or were not, picnics? In thinking about what would count as a suitable definition, a lawyer might also think about the purpose of the rule. Is it there to prevent litter, to prevent obstruction, to prevent flattening of the grass, etc. Did you discuss these possibilities? These are the kinds of things the interviewers were trying to get you to talk about. [Apologies to Scots Law who posted while I was composing.]