New
#1
It would be unreasonable for MS to not allow Win 7/8 + 10 dual boot
This is a matter of retaining access and compatibility with much already owned software. A person receives no additional benefit to it beyond that - and that is a benefit that should stay with them.
Using a license to install Windows 7, 8, and presumably 10 on a PC associates that license with the particular PC it's installed upon, and trying to install an OS with that license on another key doesn't work because the servers at MS show that the license is already registered on a PC. If a person calls MS support, they can de-register the license from the PC its on so that it can be used on a new PC.
But in the case of a dual boot, the PC its registered with for one OS is the same PC to use used with the other desired OS.
And when it's a case of dual-booting on one PC, only one OS is used at any one time, so the license is never over-lapping its use - it is always used as only one single license running just one OS at a time.