Why does setting upload speed to '0' halt my downloads?

Caution: Setting the upload speed to '0' is not the "Right Thing" to do to.

Setting upload to '0' will get you disconnected from more and more peers, because of more and more clients are using the 'tit-for-tat' way of determining how much they'll share with you. Keep in mind that seeding well while downloading does improve your download.

Also, It isn't good at all for a BitTorrent swarm if most peers would set their upload to '0', even if you say "I will share later". That's why the 'tit-for-tat' system was invented in the first place.