Not in the USA, they don't. Not as long as the cops in question don't know for a FACT that their behavior violates established law - even if it DOES violate established law, they aren't held accountable unless a previous court case established precedent under almost the EXACT SAME CIRCUMSTANCES. As you might expect, finding a previous case in which the exact same circumstances applied is not an easy thing to do. Read up on qualified immunity: https://www.lawfareblog.com/what-qualified-immunity-and-what...

There are some other answers already regarding whether or not these rules apply to on-duty Police officers but I think you're missing the point.

There seems to be a broad based perception these days that the Police don't obey the rules regarding use of force and they are not proscuted when they break them.

This might mean that the original statement: "the police can legally ... kick the shit out of you" is technically innacurate but it is functionally accurate. The Police do not appear to large swathes of the general public to be at risk of prosecution for beating the shit out of suspects (which basically means anyone they want to).

For police officers its defined by the policy of their department because they have training, less-lethal weapons, and are (at times) obligated to initiate force on subjects. The reasonable person in this case is a law enforcement officer who has the requisite training and experience.

> As far as I'm aware these definitions apply to police officers

They do, the problem is qualified immunity rules out most civil liability in practice, and relying on criminal prosecution has problems because of the relationship between prosecutors and law enforcement.