Because most protests are about making the lives of normal people miserable until they're pissed enough to hold someone to task.
Are the Tories gonna care that Stonehenge was dusted, from in their mansions? Most of them would love to dig up the stones and use them as new countertops, simply as a bragging point at their parties. But normal people that care about landmarks like these will be pissed and, maybe, bitch to the government.