The matter is that the preposition "на" is a 'werewolf' in the figurative sense of this word -- it has two different meanings. The first meaning of this preposition is 'on', just like in your first example: "У меня старая квартира в доме НА реке Мойке" -- 'I have an old apartment ON the river Moika'. When "на" is used as 'on', the noun following this preposition should be put in the Prepositional case (на реке, на дороге, на дереве... etc.). But there is also the second meaning, which is 'to'... just like in your second example: "У меня прекрасный вид НА реку Мойку" -- 'I have a beautiful view TO the river Moika'. When you use the preposition "на" as 'to', you should put the following noun in the Accusative case (на реку, на дорогу, на дерево... etc.). So, that is the reason. :-)

http://www.italki.com/groups/4151/topic/44329.htm See "Do not confuse!" paragraph.

Hi! The usage of the cases doesn`t depend on the sentence`s structure.It depends on the usage of the prepositions which follow the nouns paradigm rules.The place of the word in the sen-ce is important but all the links between the words (grammar and meaning) are mostly shown with the endings of the words.

В предложении "У меня старая квартира в доме на реке Мойке" указывается месторасположение дома. Дом стоит на чём? На реке Мойке (на земле, на песке, на озере и т.д.). Это - предложный падеж. В предложении "У меня прекрасный вид на реку Мойку" речь идёт о виде из окна дома. Вид на что? На реку Мойку (на озеро, на мусорную яму, на Красную площадь и т.д.). Это - винительный падеж.