OK, you synopsized the ruling, and you agree with it. I concur, but only insofar as it was well-based in precedent, including the always-credulous approach to the "harm" done to some individuals by the rock.
SCOTUS doctrine doesn't recognize the harm the doctrine itself does. However, I must in a few words try to convince you of it (fat chance). Typically, when Christians describe this harm, they use certain phraseologies which trigger flat rejection in the typical civil libertarian. I will try a different tack.
Once upon a time, there was a country peopled mostly by Christians. There was no nationally established sect, and citizens of any persuasion of conscience had equal standing. There were no restrictions placed on privately or publicly honoring traditional religious observances.
You see, it is possible simultaneously to have a shared common religion, and still extend free exercise and equal rights to those who follow variations of it, different ones, or indeed no religion at all.
The founders of this country and the leaders to this day openly pray to their God to vouchsafe the keeping of the country. This praying hurts noone. At times, minorities - in this case religious minorities - do feel like outsiders. This is generally true everywhere all the time and no sane person would imagine that it can or should be prevented.
All cultures everywhere have customs, many if not most retain an explicitly religious overtone. In every culture everywhere, it is necessary for immigrants to assimilate somehow. In this country, millions have participated in Christian-based customs. Assimilation has gone well.
Schools and government offices are not sharply distinguished from private concerns in the nature of their observance of such customs. To do so would be very strange, since government is by and for the people, not some separate feared entity.
The laws of this country grew organically to support this whole state of affairs. After all, legislators are closest to the people.
When the high court wades in with an agenda to change the culture, in this case to remove all signs of religious observance from state properties, it drives a wedge between the people and their state offices and schools. And it makes them feel that the safety of the country has no longer been vouchsafed to their God.
I will care if they start trying to indoctrinate my kids in school though -- you people have churches for that kind of stuff. It's the same with sex education, they're both better taught at home where the family can do it the way they want it done.
FGS