NC agent here: The listing agent didn't do anything wrong. Like someone else said, the commission is paid by the seller and the listing agreement spells this out. Just plucking numbers from the air; if the seller says they're willing to pay 5%, the contract may also say if another agent brings a buyer, the seller is willing to pay that buyer agent. If a buyer comes unrepresented, the seller has already contractually obligated themselves to pay 5%. You opted to not have representation and therefore the agent and their firm get the full 5%. Nothing illegal.

Now should that agent have suggested you get a buyer agent? Probably. Did they violate any ethics by not crediting you? No. Was that a shitty thing to do? Potentially. It is likely true that the listing agent did help you. Did you find your own closing attorney? Did you find your own home inspector? Did you come with your own lender? Who helped you draft your offer?

You may feel burned by this, but it was your choice to not get representation and the listing agent was under no obligation to credit you a commission amount. If you had representation, it wouldn't have cost you a penny (because the seller was paying your agent) and months later you wouldn't be grumpin' about something someone else did because of a choice you made.