It's still a prepostion there, but it's difficult to explain why we use certain prepositions with certain words. we talk about collocation, which is basically that words combine in patterns and combinations. These patterns don't necessarily have an obvious explanation. It's one of the areas of language where it's a question of accepting that this is the case and trying to learn them, I'm afraid. The good news is that if you get the preposition wrong, it won't normally cause comprehension problems. A very good site for examples of words in use, which you can use to check is
http://view.byu.edu Enter a word and it'll find lots of examples. You can also choose Preposition and it'll only show you examples of the word followed by a preposition. You have to register once to use it, but it's free and very fast.
"on", "in" and "at" 
Linear Mode
