The license choice is explained with the following:
> [...] we accommodate the business need for proprietary kernel modules. Unlike GPL, the MPL permits the linking of MPL-covered files with proprietary code.
Glancing at the readme, it also looks like they are treating it as a big feature:
> Asterinas surpasses Linux in terms of developer friendliness. It empowers kernel developers to [...] choose between releasing their kernel modules as open source or keeping them proprietary, thanks to the flexibility offered by MPL.
Can't wait to glue some proprietary blobs to this new, secure rust kernel /s
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