In Joe Egginton
wrote:

If crossposting is frown upon, why is it possible?

USENET is fragmented, with more than one newsgroup covering a single
topic area, or a topic may involve several interest areas.

Did a troll invent the crossposting mechanism?

No, but they use it.

For example, they might post to two newsgroups, like uk.sci.weather and
uk.politics.misc, with a topic that's completely unrelated to either one
of them.

--
St. Paul, MN