I can only express my personal view, I haven't even read the license agreement of CloneDVD, which probably says something different...
As long as you don't use CloneDVD at the same time and as long as these different PCs are not mainly used by different persons, all is fine.
E.g., you have a computer at home and a laptop. You can't use your home computer while you're on the road with your laptop at the same time, so it is perfectly ok to use the same license for both machines.
However, it would be unfair to have three machines running at the same time to clone DVDs. This would indeed require three licenses.
I avoid software which is tied to some specific hardware, and I make my purchase decision on this. I didn't buy Sonic CinePlayer or NVidia DVD Player (both outstanding products) and went with Cyberlink PowerDVD because of this.
I liked MacOpener, but I bought MacDrive because it was not locked to a specific computer.
Same for music: I want to be able to copy an audio cd, so I can listen to the copy in my car. This is perfectly ok, I cannot listen to my stereo at home while I am driving. So I avoid copy protected audio CDs. But this is drifting off topic.