alt Hacker Newsseeding is not the only way you actually upload
you're uploading before seeding, and i'm willing to bet Meta weren't seeding but, as they correctly stated in that regard, they're sharing even when they try their best not to because of the way the protocol works as zero-upload is typically impractical for any significant size files
some trackers will additionally penalise you for not sharing file parts, but this depends on the tracker
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"tit-for-tat" trading of chunks only happens between peers that both are actively downloading. Seeding nodes just let anybody leech.
You totally CAN disable all uploads in the torrent protocol. Just set the "upload budget" to zero in most clients. Just nobody realizes they can do that.
Bittorrent is wildly successful in part because every popular client makes it nontrivial to "opt out" of it's more socialist components (chunk trading, DHT participation, seeding by default).
Making an "leech behavior only" torrent client is straightforward and viable.
and the protocol doesn't enforce you upload anything.
The original design called for some kind of tit-for-tat algorithm, but it's long obsolete and you get whatever bandwidth the seeder has.