I don’t get the reasoning. Why should DDoS require that the bills do not get paid? I am pretty sure activists have done it before without bot networks.

Because effective DDoS uses massive bandwidth that you don't just have access to as an individual or company or whatever. No one needs so much bandwidth so no one would get so much bandwidth is my educated guess.

"A distributed denial-of-service (DDoS) attack is a malicious attempt to disrupt the normal traffic of a targeted server, service or network by overwhelming the target or its surrounding infrastructure with a flood of Internet traffic.

DDoS attacks achieve effectiveness by utilizing multiple compromised computer systems as sources of attack traffic. Exploited machines can include computers and other networked resources such as IoT devices."

-- https://www.cloudflare.com/learning/ddos/what-is-a-ddos-attack/

So I would say citation required to "I am pretty sure activists have done it before without bot networks". I tried searching for "DDoS without botnet", but all I find is search hits about how to do DDoS with botnets. And I am talking about effective DDoS. Not DoS.