Your point about petitioner not needing to have relationship with sponsor is valid. However, would you sign paperwork for a random person? I wouldn't. If somebody wants to charge for it, there's just high possibility they're charging several people and committing fraud.
See related, not exactly same, but you get an idea:
https://www.cbsnews.com/news/us-migrant-sponsorship-program-paused-cuba-haiti-nicaragua-venezuela/
I wouldn't want to pay $500 and discover the" joint sponsor" sponsored 100 other people and doesn't really qualify based on income.