In fact FSF specifically exempts special purpose hardware like microwaves from its purview. The philosophy is targeted at software the user has a choice to install. If the hardware provider does not intend the user to choose to install an alternative version of the system software, software freedom doesn't come into play.
Which honestly I disagree with. Tivo didn't want you installing alternate OSes on their device and neither did Sony. Alternate OS support was eventually removed from the PS3. As to the microwave, you've not had any of them do anything annoying, like beep annoying at the wrong times, or wanted a button or override beeping in the middle of the night to not wake up other people? why can't I want to install an alternate firmware to my microwave or my TV. My soldering iron supports that.
https://www.fsf.org/campaigns/free-bios.html