For security a lot of (most?) new aircraft have Cabin Video Monitoring Systems (CVMS). Depending on where you are seated they may be able to see everything you do or just your head. An aircraft shouldn't be considered a private place, as you are probably already being watched.

In 2008 a study was carried out that attempted to use facial recognition to identify passengers for signs of terrorist activity [0], so maybe they are used for that.

On the other hand, as you say it was probably just cheaper to use an off the shell Android tablet that has a built in camera...

[0] https://www.newscientist.com/article/dn14013-in-flight-surve...