Phones used to work like that too. I think all my feature phones and first Android phone worked if it was plugged in with the battery removed.

The thing about the TI powerbook was, it didn't have to be plugged in to hot swap the battery. You could do it when running on battery power. (It had a tiny internal battery, that could run it for like 20-30 seconds, so long as the screen was closed).

It did also run when plugged in with the battery removed, which is good 'cause the battery eventually failed. So this way I can still run it.