For IBM, yes. They do, but only for the parts they support. They buy support for the rest.
I work for a software manufacturer. We internally support the libs that we ship with our products. Usually this just means getting the latest source drops and building them. When there's been a problem we could solve it in our code, but being able to debug into the lib code was the only way to find it.
For major system components, and packages that aren't part of our product, we buy support.
For the average company, there's no way it's cost effective.