Agreed. It is not apparent that these cops had any reasonable cause to suspect that Ms. Daly was in possession of alcohol. Unless they knew her, I doubt that they had any reasonable cause to believe that she was too young to legally buy beer anyway. They simply don't appear to have had enough probable cause to detain her at all, much less attack her in the violent manner that they did.
And even if they knew she was under 21, and even if she did openly have beer, that sort of attack is simply far out of proportion to the crime.
Public servants should never be considered immune from criminal charges, for crimes committed in the course of allegedly doing their jobs. To attack a young woman in this manner, without having much better cause than they had, that she was involved in much more serious crimes than that of which they claim to have suspected her; is outside of any legitimate law-enforcement authority, and they ought to be charged and prosecuted criminally just as any other citizen who attacked a young woman in this manner would have been prosecuted. At the very least, they ought to be charged with aggravated assault, kidnapping, and destruction of property; and if convicted, they should spend the same time in jail that anyone else would spend for these crimes. If one of them actually drew a gun in the course of this incident, then the charge should be raised to assault with a deadly weapon.