Why didn’t they just reject the PR and not allow the agent to join?

They did, but decided to mess with them first.

A sensible human operator would have given up or questioned their premises. The agent never could of course.

Reading the article made me feel slightly uncomfortable.

There is a slightly cruel streak that can emerge in online communities - let's see how much we can mess with this and cost it money.

Without any thought there might be a human being that is impacted.

And that is a good thing. What the human operator did was completely irresponsible and malicious, paying a small bill is hopefully educational and will correct their behavior going forward.

Having agents like this interacting with human communities is a scourge that must be prevented. With every passing day my longing for a Butlerian Jihad grows firmer.