As far as I know, setting upload speed to zero disables the limit. You can set it to be very low but not zero.
That is client dependent. On rtorrent, there is a separate "off" setting for the speed throttle that means "no throttle" with the result that "zero" actually means "no uploading".
You can patch it so zero means zero.
alt Hacker News
That is client dependent. On rtorrent, there is a separate "off" setting for the speed throttle that means "no throttle" with the result that "zero" actually means "no uploading".