Might be something to do with the fact that the uploader usually doesn't own any rights to the programme and is therefore illegally stealing and uploading/downloading something that doesn't belong to them.
They're not even supposed to be uploading them in the first place, but the reason they are against others downloading from them is because a) they're the ones who went to the effort of transcoding the analogue material to digital and then uploading to Youtube b) if there are copies of the original YouTube copy of a video file, then some who download them will then upload their downloaded copy - thereby making two videos of the same video on Youtube and thus doubling the risk of both the uploaders of that video file getting caught and their accounts, some with many hundreds of rare videos, taken down.
Well Sooteries does this as well with his sooty vids.
Personally if your that worried about people downloading your vids dont upload them in the first place. Not sure why people would dl it then reupload people normally dl it so they can burn it to dvd.