I am sure that the cops have rounded up some random rowdies - whom you would probably expect to get into such situations - but were nowhere near the girl when the incident happened.
When any such episode catches media fancy - especially when you have visual proof of the ugly nature of the incident - there is intense pressure on the police to bring the culprits to book. In this case, the couple have not even registered a police complaint. The pictures are grainy, so it's going to be difficult to find the actual perpetrators.
So, I think the police have just rounded off some street side goondas. Once the media abandons the story, the guys will be out.
This reminds me, I had been the Gateway of India some time on new year's eve 2001 (or was it 2002), with family and friends (all girls) and it is extremely rowdy out there. We decided to scoot just a short while before midnight and fortunately didn't get into trouble.