Who said it was Nest's?

Maybe it runs on Oracle. Maybe it has third-party stuff licensed for their use but not open source distribution.

The really hairy stuff is usually not the first-party proprietary stuff (e.g. "This stuff is so important to our business we need to keep an exclusive license.") It's usually possible to make an argument to the right business-person that the cost is not so high when compared with the extra brand goodwill and free maintenance.

Rather it's the third-party proprietary stuff. Someone else owns proprietary code you deeply rely on but doesn't share any of your business goals and needs. And when dealing with hardware, there is a whole raft of firmware and drivers that could be licensed this way.