What counts as "open source"?

We often run (open-source) Linux on a (closed-source) Intel processor and even a lot of diehard FOSS warriors are okay with that.

Arduino Unos are a bunch of (open-source) wiring and components plugged into a (closed-source) Atmel microcontroller and people seemed to be okay with that.

How does an ESP8266 differ from an ATMega in this regard? If we consider it a basic component just like the ATMega, it's not particularly any different.