vulnerability – Why are I and other users of my ISP being used as a proxy server?

I recently noticed a huge amount of downloads made through my IP (iknowwhatyoudownload.com). On the website abuseipdb.com I discovered some reports of abusive behavior.. So I started an investigation with other users of my ISP and found out that everyone has the same problem. So I wrote down everyone’s IP and found them on public proxy server lists on dedicated internet sites. So I tried to connect to these servers myself and it was something extremely simple: I just typed in the IP and used port 8080 and it immediately worked. Initially I suspected some malware on my PC, but after seeing that so many other people are being used (maybe all of this ISP’s customers), I believe it might be a different situation.
Does anyone know what the possible explanations are?

Thank you!