Commercial reasons, most likely. The margins on selling phones sans contract are thin and if they can encourage people to go on a contract instead it also provides them an ongoing revenue stream, as well as boosting their subscriber numbers. There can also be the aspect of the stock (especially with new flagships), if you have a phone that is in demand (and the stock is tight), it makes sense to sell the SKUs you have to contract subscribers, as opposed to selling it on its own.
The price that Samsung offers it at is irrelevant - companies are free to price their stock as they so wish to either be above or below any MSRP.
The above can be applied to any form of consumer good. Why is milk in Tesco Express more expensive than in a larger Tesco style store? Comes from the same cow, right. 😀
