^^^ Well it appears that she did arrest them both...
Yeah, but only after she'd stood there and done nothing while they smashed or destroyed a lot of stuff. And only because, implicitly, they continued to smash stuff after
she'd said that the next smash would get them arrested, implying that she wouldn't have arrested them for the previous acts of second-degree criminal mischief. And there was no later indication that they were in any legal trouble for their acts.
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. Finally when the guy showed up and it became a smash-fest Beckett knew she wasn't going to get anywhere with them unless she flexed her police muscle. 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. I imagine the paperwork involved in processing those two art smashing lunatics took a while.