why online offers are only available for US reseidents?

Answer 1

Some companies do not want to take on the added expense of shipping internationally and some products here in the states can not be exported out of the country....

Answer 2

Not all online offers are for US residents. Each company decides for itself how it will sell it's goods, some small companies in the US can't afford the risk of shipping things overseas and some laws prevent some things (especially anything dealing with encryption) from being sent from the US to anywhere else. Basically it's up to whoever is making the offer.