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Because you are – effectively – buying only the print version. There is a marginal cost of pretty much zero for the eBook, so they offer it ‘for free’.
I’m sure they have some sort of deal with VitalSource technologies. Also, they have years of information seeing how many people picked print vs online so they must have figured it was worth it with the deal they worked out to provide everyone with the online version for exams 2014-on.