'Twas brillig, and the slithy toves did gyre and gimble in the wabe

It does seem odd that ARM is more heavily restricted. The conspiracy view would be because MS think they can get away with it, which they probably can - they don't have a monopoly on ARM, and it's no worse than what the other major end-user platforms do (e.g. Apple iStuff). There also isn't yet such a thing as a standard ARM platform one can just feed a CD or similar to and install Windows/Linux/whatever.

On x86 at least, I think the intention is that if you want to use secure boot with your own kernel then you'll be expected to generate a suitable certificate (self-signed would work) and use that. Since the average user isn't going to a) want to do that, or even b) know how, Fedora have chosen to ship kernels signed with a certificate that is in turn signed by the MS certificate as then they have a kernel which will work without requiring any BIOS changes. I think they intend other distributions to either get their own signed certificate (Redhat and CentOS will likely do this), use a self-signed one and require the user to install the certificate if they want to use secure boot, or just expect the end user to deal with it all (Slackware will likely do that).