I read many of the other posts. Judge much folks? I'm guessing you all have compliant kids with good coping skills. Good for you. I'm sure it's all about your stellar parenting skills, eh?
First, if you and the other parent aren't willing to hover over the kiddos during the playdate, then you should probably end them for a while. No problem with that.
But, some kids are biters (hitters, screamers, etc.). For some of them, it's part of their temperament (intense, spirited, slow to transition kids). It can take a long time and a lot of work to get them to figure out alternatives to those unacceptable actions. They internalize a LOT of stress just from minor changes in routine (getting the wrong-colored cup), and the stress sometimes comes out as a scream or a hit or a bite. Is it "acceptable"? No. But it takes a lot of work and time to help them figure out how to regulate their stress, remove themselves from stressful situations, and react in a more appropriate manner. 3yo is pretty young for them to have mastered these skills. 5 or 6yo is a more likely age, and I wouldn't expect them to be perfect even then.
Do I sound a bit frustrated? Yes. I have a spirited, intense, extremely averse to transition kiddo. Luckily, she doesn't bite. But she does very occasionally hit when she feels threatened. (And she often feels threatened when there is an unexpected transition, no matter how minor.) Her most common reaction is saying mean things or screaming/tantrums. Thank goodness she's extremely verbal (even disrespectfully so), because otherwise her outlet would probably be physical. Am I a bad parent because I "let" my kid do these things? Certainly on occasion I'm a bad parent, but it's not why she does these things. We're getting help on several levels. But it's slow.
So please lay off the judgement.