I am not talking necessarily about this particular instance. As to why, it usually facilitates breaking up the protest quicker, justifying the force necessary to make the desired arrests but it can backfire and when it does you get these kinds of results!
Also hmm, societies are built on the idea of individual and collective responsibility. Individuals are responsible for their own actions, but society as a whole is a collective of individuals thus for the system to function correctly and indeed when the system functions correctly criminality tends to be quite low!