You can do the same thing with a closed source partnership. With the GPL, anybody can take your code and resell it without modifications or with very minor modifications. No development costs means lower prices. Not only do you have to share your business, there's also someone who can undercut your costs by simply taking your work for free. No one forces him to actually make it better and give it back.
Why? because their competitor benifits from their hard work and gets their edge... but then anything their competitor builds on top of that has to be open sourced, so the first company gets the benifit of their hard work and their competitors hard work. The result is a better foundation for the software, and an edge against the companies that don't participate. The money is in support and value add in this situation.
Well, they GPL'd their byproducts. I'm sure I could convince my boss to open source our inhouse plant watering plan. But you will not see Intel giving their chipset blueprints to the public so everyone can produce their own Pentium. Not even if modifications have to be commited back to Intel. That's their product, their R&D costs and their profit. And they'd rather sell their CEO's mother than their profit source.
Thats kind of a broad statement. I know of a few business that do like it: Intel, Texus Instruments, IBM, QNX, to name a few.