First of all, it was not DOS. It was a "DOS boot disk", not a "DOS install disk". A DOS boot disk is NOT illegal to distribute, as it only contains the essential 3 files to boot the computer (IO.sys, MSDOS.sys, and COMMAND.com), and possibly will also have FDISK and FORMAT. These specific "baseline" files are not protected by the part of the license agreement on not redistributing. The license agreement only says that the whole "suite" may not be distributed (the whole set of disks, that together permit the installation of the entire package that is called "DOS"). Distributing the entire set of DOS installer disks (that has every last feature and program that comes with DOS, the entire suite that collectively is called "DOS") would be illegal. You can download DOS boot disks at many places on the net (there are even websites specializing in ONLY boot disk images, and no other kind of files), but full DOS installation disks can ONLY be found on PIRACY websites. Websites that uphold the law only distribute boot disks. The entire install disk set is always found only on websites that violate the law.
A boot disk image will have at the very least the above 3 mentioned files, and possibly also FDISK and FORMAT (to allow making an unused harddrive bootable). Distributing this is best described as "distributing a DOS boot disk", not "distributing DOS". This is legal to distribute.
An install disk image set will be a set of disk images that not only permits the computer to be booted (and possibly making the harddrive bootable), but also will install every program that comes with DOS (the DOS shell GUI, and a crap load of other stuff, that spans multiple disks). This would indeed be accurately described as "distributing DOS", which most certainly is NOT legal.
As for DOS compatibility with PCjr, if it is only compatible with DOS 2.1 to 3.3, why is my DOS 6.22 disk image successfully booting DOSBox in PCjr mode?