Embedded systems tend to need everything, including the kernel, linked together into a single binary.
But Linux doesn't, and the stated reason for not using Linux is because the license would require that the kernel would require the app to be separate. There may well be other technical reasons you don't want to use Linux, but this licensing argument doesn't seem to hold water...
DRM licences tend to further require that licensees make it impossible to write a "Total Recorder"-style program for their products, a promise we couldn't make if the kernel were modifiable by all.
except for that.
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Bitt Faulk