Hey, I figure they're thinking: "this user lets me download from them so I won't ban him". Even if you did turn uploading to the minimum, I don't think you'd get banned because:
-even if the others downloading from you can see what kind of connection you have (Cable, modem, blah blah...), there's no way I know that would allow them to see how many uploads you have and since many uploads mean lower speeds for the ones downloading from you... For all they know you could have a thousand active uploads.
-if we banned those who upload, then the network would die as all that would remain would be users behind firewalls and/or users that have disabled uploading altogether.

In short, no, I don't think that you'll get banned for having a bad upload day.

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