PokerStars Play is free to play, which means you never have to make a real-money purchase if you don't want to. However, there is the option to purchase more poker chips and chips to use on their amazing slots selection should your bankroll run low or you want to play for higher stakes.
Fortunately, in Penal Code Sec. 337j(e)(2)(D) we're told that games played with cards in a private home where no one profits from the running of the game are 100% fine. So yes, since online poker is played at home with the only gains being your winnings, it's perfectly legal.