Most major poker sites have detection protocols to prevent bot play, but some of the more sophisticated bots are able operate undetected, and some poker sites simply don't care about bots since their play produces rake (though, they will ban a bot if they catch it, or if it's reported).
PokerStars, like other online poker platforms, uses a random number generator (RNG) to ensure fair play for both real money and play money games. The play money algorithm is not designed to be rigged to make players buy more play money.