That's not illegal. So long as they have probable cause, and identify as LEO, the police don't have to tell them why they're being charged. That happens at arraignment. They don't even have to read the suspect their rights, so long as they don't ask them any questions (and if they have video evidence of the person doing something, they're not going to bother reading rights or asking questions at all. They're just going to wait until arraignment).
The person arrested can challenge that if there is no probably cause (with the right attorney, there is money to be had there) but most don't because ... there's usually probably cause.
The person arrested can challenge that if there is no probably cause (with the right attorney, there is money to be had there) but most don't because ... there's usually probably cause.