

Historically, Usenet clients tended to respond to both groups in response to articles posted to multiple newsgroups.
This tended to result in trolls doing things like posting “I’m in the market for a computer. Which is better, PC or Mac?” to comp.os.ms-windows.advocacy and comp.os.mac.advocacy with the intention of starting flamewars.











Actually, whether or not it’s permitted is, surprisingly, an undecided point in case law.
The case law here is Goldwater v. Carter, but the Supreme Court ruled on a technicality rather than the major question.
https://en.wikipedia.org/wiki/Goldwater_v._Carter
EDIT: I’ve brought it up before because a somewhat-analogous issue was also surprisingly undecided in UK case law, and there was a major legal tussle in the UK over it, whether or not the Prime Minister had the power to withdraw the UK from the EU without going to Parliament.