IoT -> real world consequences
Hmmm, I'll admit I am that sad bastard that read ENC28J60 and immediately thought "but that's a Microchip 10BASE-T serial part". Not being familiar with Azure Sphere I couldn't see the relevance.
Looking into it I can see why they may be hesitant. It's the kind of thing that could brick a device during a firmware update. On a PC or server they have got used to getting away with it, the denials of liability in the EULA may or may not hold legal water but when any mess is generally easily recovered there is little motivation to test that in court.
In lower level embedded stuff there may not be the same recovery options. That's a particular concern for something using that sort of chip since it's not something I'd expect to see in mass market stuff. 10BASE-T these days is short run niche or even bespoke stuff. We're not talking fridges or toasters here but industrial plant and instrumentation. Pricey stuff with potentially pricey consequences if it goes wrong.