I suspect one reason is that they get process a lot of chargebacks when people's fake Viagra and counterfeit NFL jerseys get seized by Customs before delivery.
Comments which few people will notice are also discouraged.
I think this is a feature, generally, but I get what you're saying. With any criteria you present for deciding what is worth posting you run the risk of it being the only criteria. Ideally, we'd all only care about our karma insofar as it helps us avoid making lousy posts, and ignore it entirely otherwise, but idealism doesn't scale.
"How will this influence my average?" is definitely an oversimplified heuristic for deciding what is worth contributing, but it's reasonably accurate and I think it's the best one we've got. (Better than total karma at the very least.)
As I said in the above comment, the voting on comments is the closest thing we have to an indication of their quality. It obviously doesn't always reflect reality, but what that points to is that some comments--the ones that we want around here but that would bring your average down--are undervalued. Those comments should result in positive feedback (via average karma or otherwise), but they don't.
That is to say: The site has a reward mechanism (karma). There is desirable behavior that is not being rewarded or is under-rewarded (e.g. replying to less popular Ask HN posts). Is there some way to reward that desirable behavior?
Replace the total karma score by the username in the upper right of the page with the average that's already visible on the profile page.
An average is a better indicator to the user of how well they are engaging with the site. It only goes up if the user consistently posts worthwhile comments, which rewards being selective about when you post and thoughtful about what you post. Fewer and better is encouraged. Consistently good is encouraged. Lousy comments (like one-on-one arguments and "Me too!") are discouraged even if nobody votes them down.
This bridges the gap between two existing forms of feedback:
total karma (which nobody really cares about, even though it's on every page) and the voting on individual comments (which is the closest thing we have to an indicator of comment quality), and gives the user continuous feedback on how much value they are adding to the site. It makes it much easier to know if your commenting is getting better or getting worse. That is, it is much easier to tell that my average is up or down from the last time I looked than it is to detect the fluctuation in the rate at which my karma accumulates.
Even if lousy comments get voted up--as happens when, for instance, someone first-posts an opinionated quip on a controversial submission and people swarm to vote in agreement--the impact on the user's average karma (which better reflects their value to the community) is much lower than the immediate spike in their total karma (the feedback they currently see). You don't have to stop people from writing such comments--just give them a smaller cookie when they do.
Obviously you can't make anybody alter their behavior over a number, but I think most here would if given a number that they could trust as representative of the shared goal of keeping this place from sucking.
tl;dr average karma is better feedback than total karma