I don't see it as bad manners at all. While I'm sure single packs have been left by mistake, having a big pile of gear sitting at the campsite certainly tells me that it's taken.

I think part of the problem that some people have, is that leaving packs on the site isn't always apparent from a distance. Depending on conditions, people might fight wind/waves to get to a site, only to find it taken at the last minute as they pull up.

I think you'd be better served, if you took the time to throw up a tarp, or hang a pack. I often use my binoculars to scan for an open site, especially if I have too work to get there. It's not an obligation, but more of a consideration, especially in the BW where available sites might be an issue. You can see a tarp/ hanging pack from a distance.

There are always going too be people who think their choice in campsites should be the one they planned on, but other's might appreciate the heads-up from a distance.