The word "credit" has several senses in English. It appears that, in this case, "credit" is being used to mean, "entitlement to a set amount of a particular company's goods or services, typically paid for in advance of use." (See, for example, here, under #2, if you click to expand the basic definitions supplied by Google.)
Thus, as a promotion (as a marketing tool), these retailers are offering some entitlement to their goods or services, without the customer having to make a payment for the privilege.
Suppose I open a new store. Then, to attract attention, I print and distribute certificates that say, "this certificate entitles the bearer to $5 towards a purchase at my new store," that would be a promotional credit.