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Not sure about prepositions, but the articles are often omitted because most Slavic languages don't have articles.

http://en.wikipedia.org/wiki/Article_%28grammar%29#Variation...

Funny thing is, when we(native speakers of Slavic languages) read such english text with omitted articles we do understand it as intended and often we do not even realize that it's missing something.

It probably has something to do with out languages having more grammatical cases than English (which, arguably, has two, though it's possible to only use one).