It's not always a concious choice not to share files!
Some people aren't sharing because they're firewalled & this effectively stops most or all other people downloading from them. I'm sure that a lot of people aren't even aware of what the firewall icon means, let alone that others can't download from them
In some cases too, the firewall comes from a school, apartment complex etc & so the user's powerless to do anything about it...should these people not be allowed to download???
It's not always as black & white as it seems