Interesting thought.
I would say that while its not specifically written down anywhere that actually downloading an ISO is in itself illegal, the person uploading it it most certainly is breaking a number of laws. You have a valid Licence and that pretty much covers you and your rights to try to obtain a new copy of your disk... The software in itself is in Microsofts own EULA a free thing and its the licence that you are paying for, or rather the rights to use the software.
So, given that you do have the licence, then trying to download the software for that licence for legal and honest reasons are, in my eyes, a completely fair thing to do.
You will never get done for such an act, and the most that the authorities will make you do, is buy a licence, and since you already have one, its case closed before its opened.