What is this "masstagger" software and why is it so hotly debated?

I've heard a few people mention this on r/news and r/worldnews, usually followed up by accusing someone of visiting a "bad" subreddit, usually T_D in most conversations. My guess is it automatically tags users that have posted on "bad" subreddits before? I've seen a lot of backlash towards it too. Anyone got a more detailed explanation?