They're not completely capricious. Laws don't operate under the confines of a strict propositional calculus: you don't follow strict axiomatic derivations to a contradiction and declare yourself finished. Instead, it operates generally on a basis that cases with facts in common should have the same decision (hence the name "common law"). And the law is written with this process in mind: if legislators don't like the judicial interpretation of their text, they can change it by changing the law.