You are correct that it is cheaper and easier to just give back, but that is generally true and isn't unique to the GPL. There are a great number of companies

You are correct that it is cheaper and easier to just give back, but that is generally true and isn't unique to the GPL. There are a great number of companies using the BSDs as the basis for their embedded projects, and they contribute their enhancements and bug fixes precisely because it is less expensive over time to do that than to keep those changes proprietary. The USL lawsuit is what hampered the BSDs early on, and allowed Linux to get so much traction in mindshare, not the license.