The article has been misleadingly named. The article text itself makes it clear that he was fired for abuse of his discount card, not for giving discounts. There is a difference, because a salesperson may or may not have the authority to give discounts, while the card was presumably for personal and untransferable use.
Going by the article itself, I can't fault the company. It should have told of the policy early on and claims that it did. The worker denies that he was informed or warned.
Even if it turns out that the company is lying and the man is not, at the end of the day it is still a situation of extending personal benefits at the company's dime and way beyond the intended scope. His quotes in the article imply ignorance of the rules as opposed to having requested the extension and being denied, so I have to assume that he did not ask for permission.
The end result is a company that had one of its workers deciding unilaterally, without prior clearance from his employer, that his 10 to 30 percent personal discount was extensible to customers of his choice because he wanted to do something nice. It is not entirely clear whether or to which extent he took measures to hide that decision, which would aggravate the situation.
If he is being sincere, then I fear that his discernment is probably too flawed for him to be a good fit for a food co-op. If he is not, then he is acting in bad faith and the store has a duty to protect itself from his actions.
At the end of the day, he decided to feel generous by abusing his card and having the co-op pay for that generosity.
As an aside, a food cooperative is somewhat different from a grocery store. The main differences are in ownership, decision-making, and money flow. That extension of discounts would still be abusive if the owners were distant millionaires, but odds are that they were instead people much closer, cooperative members that he may well have met in person, even on a daily basis. He may have convinced himself otherwise, but he was being generous with their money, not his own.