Seems pretty clear to me
So say you owned software that you distributed
1) free - via download from your official site
2) for $10 for a physical "branded" CD (to cover costs: manufacture/print/distribution)
In both of the above cases you controlled the content available from your "official" sources. If someone does the download they know the source. If they purchase a "branded" Cd, they know the source.
Now someone comes along and downloads from the official site, then burns off a few CDs and hands them out to friends. The CDs are not direct from the official source so there is now an element of risk as to what is on the CD. Maybe the person added a few "extras" (accidentally or intentionally), but they certainly cannot hold you accountable for any malware on the CD that is not on your official version.
Microsoft has contacts and deals in place to ensure that the company making its CDs are only loading approved content. If buddy has someone in China make the CD then who knows what else may be on there. By putting official MS and Dell logos on it they make it look as though these are from a trustworthy source. Not sure people would feel as secure using "Bob's computer recovery CD".
Not saying that MS has the best reputation, but I can see why they would not want this. As several people have stated: it was the decision to "brand" this and pass it off an an official copy that was the issue.