I think that the presence of such employees depends on the size and profitability of the casino. The larger the casino, the more serious and professional its security service is. I think the biggest casinos in Las Vegas and that's where the best security services in the world are.
For me, as a player, the main thing is that I can enjoy the game and no one interfered with it.
The bigger casino doesn't want to see their reputations damaged because of the small mistake, so they make sure that they can handle their environment. Maybe they are placing their employees as gamblers, making sure that everything can run properly and make money from their place. A gambler who only want to enjoy the game and having fun in the gambling will not want to think anything. They will let the casino handle everything.
This may be possible, and there may be some other tactics could also be deployed just in order to ensure the house is in order and nothing fishy happens while gambling. Because any unpleasant kind of experience if shared in outside world then the reputation of such casinos will take a hit and would result of switch of the gamblers from one to other casinos and nobody would want that to happen.
I've heard of another tactic. Major casinos have a database of undesirable individuals who have been caught cheating or are simply suspected of cheating. Such people will be denied access to the casino. II have also heard that major casinos exchange this database with each other. And if you get blacklisted in one Las Vegas casino, you may not be allowed in many others. This is also a good protection against cheaters.
It's not impossible, they are on the same business so they are into protecting their respective businesses, though they are competitors but when it comes to cheaters they are united with that. As the saying that I heard before, when you are a casino, they will love you when you keep losing money but if you keep winning, they will hate you and you are not welcome since you are a threat to their business.
I don't know, but is it also possible for a gambler to be ban in a casino even if he is not cheating because he is always winning?