The OED is useful here: it literally defines in as equal to on in the second definition of the prepositional form of 'in'. There's an extended history of the relation between 'in' and 'on', which includes the Latinate source of the Saxon term, its evolution in Old and Middle English, and its use today. Suffice it to say, the distinction is not a trivial one, and 'in' and 'on' are often interchangeable.