Forgive my ignorance but how is the Kobo running Linux a GPL violation? In the user guide one will notice the extensive pages dedicated to providing a copy of the GPL (which they are required to do) and other information regarding other packages used on the Kobo. In regard to the source, GPL'd source of distributed binaries do not need to be made public. They do need to be provided upon request, but Kobo do not and are not obligated to go "here is the image, oh and here is the source". They are obligated however to provide the source code when asked.
Some companies prefer to just provide the source along with the binaries and alleviate any issues of people wanting to request the code (ie: they provide it regardless).
IF, and this is IF Kobo have been approached and a copy of the source code has been requested, and Kobo have failed/refused to deliver, then they could be in violation of the GPL.