"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.

Tit-for-tat kicks in. It's fine for smaller files to just jump peers with zero upload, but i reckon Meta would have found it challenging to download very large files without sharing. It's certainly much faster if you don't get throttled or banned by many peers.