Comment by smokel

If they use unmodified Linux, then they only have to provide (a link to) the source code to that kernel on request. No source code is required for proprietary add-ons, unless they are kernel modifications.

The GPL also does not state that the source code should be easy to find. In the early days, one had to write a letter, send it by mail, in hopes of getting a tape or CD-ROM with the source code. For which you then had to pay as well.