My thinking is that since she was investigating a murder, Beckett didn't really care about what the woman was doing since it didn't endanger anyone else, she just wanted to get some answers about the case.
She could've at least mentioned that it was a felony. After all, if she wanted the ex-wife to stop smashing stuff and focus on answering her questions, the most efficient way to do so would've been to tell her that she risked jail time if she didn't stop. Sure, Beckett did that eventually, but my point is that it came far too late, that both in terms of the law and her own interest in getting quick answers, she should've threatened arrest after the first act of destruction, not just stood passively through about a dozen more.
I assusme that in real life a detective actively working a case wouldn't care too much about a driver failing to signal a turn or some other relatively minor issue in an effort to solve their case.
That's a moving violation, not even serious enough to count as a misdemeanor unless it gets someone hurt (it belongs to the lesser category of "infraction"). We're talking about a serious property crime, a felony that can get you up to 7 years in prison. Class D felonies include some pretty significant crimes
like varying degrees of assault, fraud, bribery, and drug dealing. The point is that while the episode treated it as minor, New York law actually takes it much more seriously.