Without going into too much depth with all the legal bullshit ( Of which I would have no idea ) most of the companies that originally made pinballs have long since gone with the exception of maybe Stern  and perhaps one or two others, but even so the copyrights for artworks ( playfields, backglasses etc) remain with the copyright holders and anyone doing reproductions for commercial gain have to have permission ( and usually pay licensing fees ) from the copyright holders.

I think the generally accepted rule of thumb is that if you own a particular machine and want new ROMs for the CPU boards or if you want to scan a mates backglass because yours is broken or missing then you can do it provided its for you only and you're not selling them for a profit.

Thats my limited understanding of the whole issue - Anyone else feel free to correct me if I'm wrong or add anything missed.