I tend to agree with what you are saying - it seems most unfair that people would download songs without voting for them. After all, they clearly like the song or they wouldn't be downloading it. Since they are getting it for free the least thing they could do is click the thumbs up button as thanks.
However, one thing to bear in mind is that only people who have registered an artist or 'listener' account and are logged in can leave a vote, whereas anyone who surfs into the page can download tracks.
While your idea is a good one (download option only available after voting), it would prevent unregistered people from doing either, and to be honest I don't think most regulars would want this.
I know artists who have songs with over 100 downloads but less than 20 votes, but rather than be upset they say they are humbled/flattered by this.
They see the number of downloads as a marker of a song's lasting popularity.
Conversely, there are cases where people leave votes without downloading - for example, if they appreciate that a song is very good for its given genre, but not something they necessarily want to keep on their iPod.
About lasting popularity, the total number of votes does accumulate over time in the same way as the total number of downloads, but may not necessarily have any relevance to the charts.
For example, if one of your songs got one new vote every month the total would build up impressively over time but your song would float permanently around the bottom of the top 100 (if even on it). High chart placement depends on a track getting a lot of votes within one or two weeks.
Another possible solution would be to make voting possible for casual surfers too, but this would be open to abuse or misleading counts. As you said, a certain number of downloads could be attributable to bots, not humans, and if voting were possible for unregistered surfers there could be large amounts of bogus thumbs up attributable to software, which would have no relevance to the quality of the track whatsoever.
Having tracks only votable by logged in humans keeps it real.
In closing, if you don't like the idea of ungrateful fans downloading your music without thanking you with a vote (which costs them nothing, it has to be said), you always have the option of disabling downloads on any or all of your tracks.
If you liked, you could even leave a comment in the song notes asking true fans (the one's who bother to leave votes) to e-mail you for a free mp3
It's up to you!
