I don't think this is unethical or problematic. According to the original feature request, there are essentially two reasons why viewing vote counts is a privilege for 1k+ users:
Displaying the vote counts requires a database hit, given how the vote data is stored. This is not only a performance issue but puts additional stress on the servers. Given how rarely this data is actually needed or useful, it doesn't make sense to provide it all the time. Rate limiting it to only 1k+ users helps to lessen the load—and everything here is rate limited.
It is just something we can give to users when they reach 1k reputation. Otherwise, there's an awfully large gap between the privileges earned when you reach 500 (ability to retag) and those you earn at 1500 (ability to create new tags). Show vote counts is just a nice stepping stone on the long road of privileges, and it provides a sort of incentive for new users to keep answering questions and earning reputation.
Since this information is provided as part of the API (and can in fact be calculated mathematically from a post's timeline), it's not really a problem that your extension would be providing that information. Unlike most privileges, this one is not limited only to high rep users because of the power that it provides.
...unless you believe that information is power.
This other snarky response by Jeff also provides some indication that you're not doing anything illegal.
Now that Jeff's gone, have his various answers lost all of their original luster as definitive nuggets of Meta gold? Or are we all just going to perpetually defer to his spirit? It's what Jeff would have wanted...