True, it might not be a true ideomatic expression, but I consider it one because if you were to translate it literally, it would mean "for the night", which doesn't match the true meaning and so we translate it idiomatically to get the better phrase "at night/during the night". But yes, there are other expressions that might be better classified as idiomatic than "por la noche": http://www.studyspanish.com/lessons/porpara.htm
Personally i don't consider it idiomatic at all. Prepositions just can be used in different ways than the english prepositions that typically correspond with them. The word por could be defined multiple different ways in english. It does not mean just "for". "Por la ventana" means "through the window", for example. Por could mean for, by, through... So i just consider this a normal old translation.
'por principio' also means 'on principle', 'as a matter of principle'.
Does that mean that if the definite article 'el' is included (por el principio) that we can safely assume it means 'at the beginning'? And when the definite article is omitted that it means 'on principle'?
Maybe someone can confirm this for me...
Russ, it does not! I do not support mindless word-for-word translation but there should be some basis for the translation you propose... Where does the concept of "first" come from for example? Don't say 'principio' cos you are using that I imagine for 'principle' - or as you prefer 'principles' - incidentally el principio = the (singular) noun. So you have really done a poor job of getting the meaning of the Spanish sentence.