Why? Because it's legal. And they feel like it. And they have nothing better to do but to screw around with users that don't know the difference.
Here is the article. They say they are fully disclosing at the point a download completes, and the user is presented with nail.exe for installation. But my guess is, most users want to see what they've believe they've downloaded and think they are agreeing to that, not a spyware install. MMG says it's all totally legal. But I'm also wondering, did they host the file the user really wanted, and then send their spyware along with it, or did they just slip it into bittorrent streams as they found them going across the network, or did they find sites with desirable content and seed their spyware files in with the others?
Someone hacked their site today:
But you can contact them here if you want to complain about their practices:
Marketing Metrix Group
1344 Pheasant Lane
Victoria BC Canada