Regarding Karthik's question: Do you think that way because in English, one says "fear of God"?
In Sanskrit, the reason for fear is always in panchamii (ablative). Cases, like prepositions, have idiomatic usages that differ from language to language and cannot be translated word by word or morpheme by morpheme. [According to etymologists, 'of' and 'off' are both from PIE 'apo', "away", so "fear of God", in origin, is no so different from Sanskrit. It is the use of 'of' to form possessives in English that is somewhat unusual.]
To me 'devasya bhaya-' would mean "God's fear", a very different thing.
Re Vishvas' response: The vaartika on the relevant sutra is to recognize the use of panchamii to indicate reason in all situations. [Panini allows only tritiiyaa for reasons in general.]