While this certainly happens, there are companies that do not want their codified knowledge to leak to competitors. So they lock down the IP not because they want to sell it, but just in case code happens to contain trade secrets.
My terms of service account for that. You need to separate the business specific from the underlying technology in different deliverables and keep the IP for the underlying technologies yours.
I tend to reuse a lot of code from previous projects in new ones, so this is essential.
