There’s an element of intent which your sarcastic response doesn’t really account for. But yes clearly everyone who wears a helmet to a protest does not intend to start a physical attack.

That doesn’t mean it’s not an effective preventative measure and de-escalating technique.

Police standards and the populace's standards of what constitutes legal gathering and protest are often quite different, as evidenced by the unnecessarily harsh deployments against protesters as seen in Hong Kong and less recently in the US. More often than not, police forces make the first move in otherwise legitimate protests either with direct oppression, false-flag tactics, or unnecessary displays of force (or potential force).

A helmet is entirely necessary in peaceful protests because the authorities resisting the demands of the protest do not play fair.