There is also an important distinction between prop players and shills that was touched on but really needs to be clear imo.
Prop players are paid hourly to play by the house/casino. They play the game using their own money, they keep their all of their winnings and suffer all of the losses. Also, they don't always get what they want. Often times prop players are put in short handed games to fill the table or they are used to start new games, etc. Not always ideal for them but that's the point of the hourly compensation. In a way you could look at their hourly wage as sort of a rake back because they earn that from the house while playing and whether they win or lose.
A prop player is HUGELY different than a shill. A shill is put in a game, gets paid by the house, plays with the houses money, and essentially is trying to win money back for the house. So basically using a shill is a house/casino trying to screw their players not just by taking rake but also by trying to win their money in the game as well. Generally using a shill is immoral/unethical at best and, in some areas, illegal as well.
Finally, to the best of my knowledge no legit online sites use shills and most do not even use prop players. Last I saw PokerStars
(as just one example) specifically states on their website that they do not use prop players for any reason.