It is marketing, simple as that.
You run a seaside cafe that is quiet during the week out of term time, so you need to attract people in. The cafe 5 minutes walk away does an OAP discount, because OAPs are generally the people out and about at the time you need customers through your door. Do you give a similar (or better) discount to get more customers, or not bother and make a loss that morning?
The question really is, should private businesses have the freedom to offer discounts to any group they like, in order to market their business and increase income? Surely the answer is yes, so long as it doesn't disadvantage any protected characteristic, such as a white skin only discount.