Not sure why downvoting would require a minimum point score other than to discourage lurkers (like me) to lazily engage in discussions (even though I believe voting is in fact engaging). This added to the fact that the HN crowd is notorious for downvoting unpopular or even slightly pessimistic comments sounds to me like forced positivity.
I presume it's to make it harder to create sockpuppet accounts.
Also, reaching those 500 points requires quite a bit of contribution, so there's a good chance that you understand the rules by that time. I know the downvotes here can be annoying, but in comparison to Reddit, for example, it's very calm, probably in part due to that limit.
The -4 maximum is there specifically to limit the impact of that. I'd say that 70% of my comments which stay in the negative, I earned it, and the remaining 30%? Well, you can't buck the hive-mind: there are simply some things I believe that most people disagree with, and my choices are to shut up about it or take my licks.
