Torrents are based on the idea that everyone using them pays for it with their bandwidth and hardware cost. Except for those leechers who don't share.

I'm paying more for my seedbox than for my usenet subscription. If I used my own hardware I'd pay with stress on my hardware, e.g. the disks aging and failing earlier because of seeding. The power consumption is also not negligeble, altough the server is also used for other purposes.

With private trackers this idea of an equal exchange is more obvious because of ratio requirements.

Edit: I'd say it's similar to open source in that no single individual has to pay for it, but someone does have to, for it to exist. Most often with their (valuable) time and knowledge. If no one helps out and does their part (through money or time+knowledge), a project won't survive for long. Same is true for torrents.