After reading this, and other people's response, I am in the "I guess it depends" camp. As in, I guess it depends on WHY she is crying. Sometimes I see other parents disciplining their kids in a way that seems rather harsh and severe but I think about all the times I've been out in public with my daughter (soon to be 4) and I've had to get her stop some behavior that was not appropriate and I probably sounded pretty harsh too to some people. Usually it was some whining or crying that I just did not have the patience for after dealing with it multiple times already, all day long. Maybe your BIL and SIL are just trying to prevent their daughter from being a "drama queen." Maybe they have to listen to her cry at the drop of a hat over every little thing at home all the time and they have decided that they have had enough.
There's crying because you have hurt yourself. There is crying because you are genuinely sad about something. And I am fine with that. But there are days that my daughter starts up about everything that just doesn't go her way. We are out of raspberry yogurt. Her Legos won't fit together they way she wants them to. Her favorite shirt is in the wash. She decides she wants to bake cupcakes and we are not baking cupcakes today. Daddy ran to store for 10 minutes and she is going to miss him. And honestly, there are days it just gets to be too much, and I just have to tell her enough is enough - if she's going to keep on crying about every little thing under the sun, she can do it in her room. I can only validate her feelings so much before I start thinking, "Oh really, what now?" But there are those times I may just try to give her a hug, cuddle with her on couch, and acknowledge her by saying "Boy, you are just having a real bad day aren't you?"
Like others have said, these are first time parents, and they probably have a lot of ideals and expectations that may or may not be realistic. I think what I would be more concerned about is when they try various methods to try to get her to stop crying, but they clearly don't work, and yet they persist. If you feel that strongly about it, then by all means, say something to them. But I would give them some suggestions as to what they can try instead, rather than making them feel like they are being attacked and put on the defensive.