I'd like to be able to install my own software on all of these

I'm not asking what you'd like to do. I'd like to be able to customize all of those things too.

I'm asking why taking a device that uses a microcontroller and making a new model with an ARM chipset and a Linux-based OS seems to suddenly make people treat the ability to install custom software on it as a fundamental right.

If I own it, regardless of if it's Linux or ARM based, I should be able to install things on it.