"How could you...?" means "I can't believe you did that" and "You shouldn't have done that":

How could you say that? I thought you loved me?

How could you lose a three thousand dollar check? What is wrong with you?

When the wife asks her husband "How could you buy them...?" she doesn't really want to know how. She wants to know why he bought the tickets without asking her first. She's also angry about him doing that.