I think you are over-intellectualising it. People post cross-forum for the same reason that people tell the same joke at different parties. It's easy (you've already prepared the picture/learned the joke off by heart) and rewarding (people say "nice shot!"/laugh out loud), so why not?

I can't see that it does any harm though.

Disclaimer: I rarely cross-post between fora.

Disclaimer to the disclaimer: but I am notorious for telling the same joke to everyone I meet.

Disclaimer to the disclaimer to the disclaimer: on the other hand, for those of us with our own websites, you could regard every forum post as a sort of a cross-post. I actually often wonder if it's worth posting a picture here on the theory that it's already on my website if anyone wants to see it. But if I post it anyway, I usually feel as if I'm cheating a bit, 'cause I'm only reposting something I've already done. Doesn't altogether make sense, but there you go.