Most downloading is illegal. But there is a difference between crimes that the police will arrest people over, and crimes for which someone 'wronged' can sue. For downloading the 'wronged' party can sue for damages, which would be loss of earnings for that one sale. Plus, I guess, costs.

Uploading is easier to sue, because there the damages (the number of files made available) is much larger and it makes things more cost effective. It also (possibly?) acts as a deterrent to other uploaders.

Criminal charges tend to come in when money is being made from copyright infringement. Selling burnt DVDs, for example.

I'm being a bit vague because of the international audience. (I'm in the UK.)