Bootable Win8.1 USB activation piracy questions

I created a bootable Win8.1 USB drive.  I inserted it into my laptop, booted from the USB drive, and entered an off-the-shelf (never been installed) 8.1 Pro license key to activate.  I then carry that drive with me and use it on any computer I want in the field without having to lug a laptop, tablet, etc.  I just use whatever PC is available.  Someone asked me if what I was doing was legal, and why I didn't get activation issues when booted from different hardware.  So, my question is two-fold:
1.  Since I entered an off-the-shelf 8.1 Pro license key that has never been installed, then my portable USB is legal, even if used on different hardware, right?  If not, show me MSFT EULA that says otherwise.
2.  As far as I know, MSFT only started using motherboard registering technology with Win10.  So if I installed a copy  of Win10 on a laptop, Win10 is forever married to that motherboard; the mobo serial number is in MSFT's cloud activation database.  But that tech wasn't used in Win8.1, right?
Thanks for any insight.  I want to be legal; I think I am.  But, this is darn convenient.  And the Win8.1 Pro USB drive is only used on one pc at a time, even though on multiple hardware.