How does Cloanto know Franko doesn't have a license to distribute them?
The most obvious way would be to ask from the licensor.
Does CLoanto "own" the Copyright so do they even "have" the right to complain?
Most (if not all) contracts drafted by Amiga Inc appear to contain a clause that requires the other party to protect the rights of Amiga Inc. This could be seen in case of Amithlon for example (see
http://www.amigamccc.org/journal/0206amit.htm). While pure speculation it's not far fetched to assume similar clause in the Cloanto contract.
We don't own the copyright, so do we have a right to complain to Cloanto's ISP because we suspect they do not have a license to distribute and demand proof they do?
You can always try, but in the end your claim won't have any legal merit.
Second of all regarding Franko and a license for distributing copyright, I think you are missing the point. I think basically Franko is distributing these Workbench disks under "Public Domain".
He has no right to place them in public domain.
Who's burden is it prove these are public domain?
Franko's, of course.
I think the problem is here, it's possible Cloanto has a license to distribute Kickstart images and disks, but at the same time the license is either made by ill-intentioned crooks from AI that know they don't hold the copyright or just ignorant of the fact that the disks went into the public domain 15+ years ago.
Either way, worst case scenario, if the Workbench is still copyrighted, I fail to see how a third party that just a has a license to sell a product can act on a copyright holders behalf.
If workbench or kickstart would be in public domain it would need to be clearly stated as such by the copyright holder. Since this is not the case they are not.
If there's no valid license then no-one can distribute the work, except the copyright holder. The default is to protect the rights of the author(s).