This story does indicate that inasmuch as it's agreed by all sides that had they refused the customer service because they were gay, this would have been illegal. We were taught that it was within the rights of the store to refuse to sell something to anyone, for any or no reason, with the expected cases for that right to be used including the aforementioned drug dealers and anyone trying to take the piss with misprices.
As mentioned above, it seems to be accepted by everyone that you can't if the actual reason is discrimination against the customer on the grounds of, say, sexuality, religion or race.
