In theory, anyone can go and make Linux a non-free program, since it is simply impossible to enforce the license there.
I suggest you read http://gpl-violations.org/ in order to see where the GPL has been discussed, and upheld, in court, following companies attempting to make the Linux kernel proprietary. Harald Welte, who started the site and wrote some of the Netfilter code, took (and continues to take) many companies to court over the use of his code, which is included in the kernel. The violations being sued for now are on a wider scale than just Netfilter, as I understand it.
This is different, netfilter had presumable only a single copyright holder (or a few), Harald Welte. This isn't the case with the whole of Linux. For each infginged part, you would have to figure out who the copyright holder is, and ask them to sue. Something that is infact a practical impossibility. Harald Welte can only ask for his copyrighted bits to either be removed, or have the infringing party to comply with the license, he (or the court I think) cannot dictate what should be done with the other parts, since no copyright holder has come up and sued.
None of the cases at http://gpl-violations.org are about companies infringing on the license of Linux, but seperate projects or stand alone bits in Linux, as far as I can see.
I spoke about Linux as a whole, not in small parts.
For example you could do something like: take Linux, someone sues, you remove the infringing bit, continue distributing a non-free version of Linux, and simply wait until someone "sues" again (have any of the cases listed on gpl-violations.org gone go court?), and only do the minimal to comply with the bits you are infringing on.
Anyway, this is all in theory...
Cheers.