For the government and police, it's a cheap option to deal with the problem.
The abuse is a very real problem, I know a few who work in shops, all of whom have been verbally abused and physically threatened at various times. One of them was not long ago punched in the face in front of witnesses. Tesco, ever caring, had her working the next day, just a few hours later (incident happened on late shift, next shift was early morning, "are you sure you can't come in?, it'll be hard to find someone to cover") and seemed to put as more effort into investigating what she might have done wrong as they put into helping the police gather evidence to investigate the abusive individual, so she was glad of the CCTV. She got out off Tesco as quick as she could. She has had notification from the police that the individual involved is being prosecuted, but for a number of other offences also so it is unlikely she'll need to testify or otherwise be involved further.
The staff want the CCTV because they don't trust the managers and higher to look after their interests otherwise.