You contradict yourself a little bit. If you decide your PC is a heater, well, then it is one!
It is commercially unviable to sell it to someone third as that, but if you managed that feat, it would be a bit strange if this person then came to you: "You sold me this electric heater, but I looked into it and it is warmed by an especially hot Intel CPU. So I decided it is a PC and despite you are being in the heater-selling-business I want you to not void my warranty when I reflash the firmware so I can install Windows XP to play Solitaire in long cold winter nights."
>> I want you to not void my warranty when I reflash the firmware
That's a different issue. I support the manufacturer saying "we won't guarantee our computer-as-heater will work if you change the software."
What I don't support is "we have have made it legally/physically/cryptographically impossible for you to use the thing you bought however you want to."
If I sell you a watermelon, I'll guarantee its freshness. I'll not guarantee its fitness as a boat anchor, but if you want to try that, go right ahead.
It is commercially unviable to sell it to someone third as that, but if you managed that feat, it would be a bit strange if this person then came to you: "You sold me this electric heater, but I looked into it and it is warmed by an especially hot Intel CPU. So I decided it is a PC and despite you are being in the heater-selling-business I want you to not void my warranty when I reflash the firmware so I can install Windows XP to play Solitaire in long cold winter nights."