From apples side there is NO restriction to run windows on the M1 Macs however the issue comes from Microsoft.
Microsoft doesn't give out licenses for its Windows ARM variant.
M$ just doesn't sell retail licenses for ARM.
Why would they? There are few, if any, ARM-based desktops of laptops on the market apart from Macs. There's no real ARM equivalent to the standard "IBM" PC architecture, nor do hardware manufacturers routinely supply windows-on-ARM drivers for all their hardware. The Raspberry Pi is barely powerful enough to make a practical Windows machine, and there's a developer preview version that can be downloaded and run on a Pi if you just want to experiment. Phones, tablets etc. can't run Windows without various degrees of hacking and jailbreaking... bottom line, approx. zero people would actually buy a retail Windows-on-ARM until there's something to run it on.
Before WoA will run on M1 Macs there needs to be a
complete Hypervisor package available to buy: Parallels for M1 is still in tech preview, as is the version of WoA that actually runs on it - lots of people have tried it and reported it as extremely promising, but still unstable.
...also, the WoA x86-64 emulator has only recently turned up in the preview version, not released yet, and there's also a suggestion that the current WoA still contains bits of ARM32 code which won't run on the M1.
Microsoft "gives out" (methinks the odd dollar changes hands) OEM licenses to Dell, HP etc. to make their own Windows-on-ARM systems. Potentially, they could do an OEM license deal that enabled WoA to ship with Parallels or VMWare.
But why would we hear about that until there's actually a stable solution that works?
It's quite true that you can't buy a retail WoA license yet, and it
might never happen, but there's no particular cause to be pessimistic while there would be no
point in releasing one.
What we do know is that Apple are not
supporting booting alternate OSs (i.e. Boot Camp) on M1. They will support virtualisation. Federighi said it point blank. Quick google will give you lots of references to that.
To run Windows on Boot Camp, without Apple support, MS would potentially need to reverse-engineer Mac Os in order to write Windows drivers for all the functionality which is now implemented in proprietary Apple Silicon. Not impossible, but a lot of work, prone to triggering Apple's lawyers, and likely to result in incomplete/unstable drivers not up to MS standards. To run in a VM, however, you don't need native Apple Silicon drivers - the Hypervisor provides a virtual device which it implements using MacOS frameworks. Hence, the WoA preview is already running on the Parallels preview, complete with graphics acceleration etc.
The later Federighi "up to Microsoft" comments were referring to virtualisation and the fact that there's a M1 Hypervisor "backend" already built into MacOS. Which is true, but there's still a lot of work to turn that into a full-blown hypervisor app with virtual graphics devices etc. Hence Parallels still being in tech preview.