Difficulty Finding Tracker Host

Why is it so difficult to find anyone willing to host a torrent tracker, especially in the US? I understand that public trackers can be used to enable piracy (a la PirateBay), but that doesn't explain why private trackers wouldn't be allowed. Besides, should someone start blatantly infringing on others' copyrights, the host would be well within their right to shut them down. So, why do some many adopt blanket "no tracker" policies?