The fact that it is written in a UN pamphlet does not mean that it is a desirable practical implementation at state level.

Physically, no, religious symbols cannot harm anyone. But consider the massive impact that certain symbols have had on sets of people over the years. Many of the most violent and destructive movements of history marched under supposed religious banners, and a lot of psychological harm can be done by reminding people of hardship they have suffered by these people. The schwastika (Gershwin's Law!), for example, is a symbol taken from Hinduism, I doubt any Hindus would march around wearing one on their sleeve.

The individual, particularly the religious individual, does not exist in isolation in modern society.

The problem with a society that allows religious expression is based in the nature of religion itself. Unlike political opinions and political freedoms, religious ideals deal in the realm of "absolute truths", whereby a mentality of absolute right and absolute wrong are created. Because religion is inherently a social belief, religious groups tend to exist and view themselves as separate entities to others, creating power blocs which transmit into political structures. In combining this belief in an "absolute truth" with political power, distinctions between religious and ethnic groups are heightened, dividing society and leading to conflict and resentment. A world without public religion would encourage association without prejudices and lead to greater social cohesion.

Because the state has to take into consideration the collective good as well as individual good, it can justify action against individual beliefs in order to promote the wellbeing of a whole society.