There are many companies that violate the GPL by not sharing their modified code on redistribution. Eventually they comply on request or lawsuit (in thanks to the Software Freedom Conservancy). It's not the contribution OP is after (direct project interaction) but I consider getting access to their changes to be °giving back to the community°.
If it's one dude not contributing back I ain't that worrried but if it's a big company then that ain't good. It's doing free work which could have been paid for (if not to yourself then to someone else doing the work for pay). Also, I value software freedom so I consider proprietary software to be harmful in of itself.