I know way too much about this nonsense....I actually watched the first four seasons for reasons that might be best explained by being too tired to watch stuff that required brain activity.
I think that the hook was just not knowing who was singing, rather than not knowing if it was a professional or amateur behind the mask.
For better or for worse, the U.S. has 1) lots of people making a living singing and 2) a lot of people who had a flash in the pan with respect to fame, but a years-long career singing that did not gather widespread attention, leading to much nostalgia value (though the nostalgia is often for the last decade).
It looks like the South Korean version of the show, which spawned all this stuff, had mostly singers as contestants. Most of the other international versions of the show have a lot of professional singers perform, though occasionally someone not known for their singing places in the top 3.
David