I suppose it's a bit like the Wild West at the moment, with everyone figuring out the rules as we go along.

I suppose there are a few explanations for this, helped by the fact that it is very easy to share a post between instances and communities (it makes more sense with the latter where there is crossover, eg a band is playing at a festival and you post to the band community and then copy that to the festival community):

a) it's just someone who doesn't understand how instances work. A polite message would seem to be in order.

b) it's someone using Lemmy to promote themselves or a product (either doing it by hand or possibly using some kind of bot). Everyone's mileage might differ on this but, to me, that looks like spam and should probably be removed. I imagine when we have robust (any?) moderation bots that is the kind of thing that would ping up on their radar. For now, if you think it looks spammy, report it.

However, I haven't seen any examples so can't really go into more detail on that.