I think it should be up to the client to decide whether they want to seed

well, yes and no. legal issues aside (think about using bittorrent only for legal stuff), the whole point of bittorrent is that it works best if everyone uploads.

actually, allowing clients to disable uploading is almost an acknowledgement that illegal uses should be supported, because there are few reasons why legal uses should need to disable uploading.

and as an uploader i also don't want others not to upload. so while disabling upload is technically possible, it is also reasonable and not unlikely that connections from such clients could be rejected.

In the US, data caps are one reason to be stingy about seeding even if legality is not an issue. In that case though the user could still do something like limit the upload bandwidth while still seeding long-term, so their contribution is to provide availability and prevent a situation where no full seeds exist.