Because, no matter whether or not it's technically spam, people will stop sending such messages if they get "marked as spam" often enough. Either that, or GMail will start silently dropping their mail - either way, my inbox is clean. Making such a policy public can only help.
[Note that I don't mark such e-mail as spam, if only because there's no convenient way to do so over IMAP.]
I feel like the only reasonable response, then, is to start madking all email sent from people who believe this as spam, so they can get a sense of what this feels like.
Seriously, though: people who abuse the system like that,--whether they are giving products bad ratings for reasons unrelated to the product, using the PayPal dispute system as a threat (or seriously on everything they purchase from everyone online as it let's them get stuff for free), or downvoting comments on HN out of retaliation for a lost argument--need some kind of meta-moderation to keep their data from affecting the system.
[Note that I don't mark such e-mail as spam, if only because there's no convenient way to do so over IMAP.]