The civic space should be equally accessible to all members of the community, since it is owned collectively by that community.
Philosophy and ethics are determinedly private matters, and cannot be readily translated into public language that we can all partake in. It isn't about removing contentious symbols that might offend some people; it is about making sure our government does its work in a language we can all speak. (See John Rawls for more about the distinction between private and public language.)
Here's an interesting thought: Why don't we think of the concrete absence in the public space of symbols that are religious or similarly "private" in nature, as a monument in itself? A monument that says something in its silence about our solemn shared commitment to a form of government in which persons of all and any creed can all participate equally.
(That said... if any viewpoint is given space to advertise, then all of them should. Let the reductio ad adsurdum show policy-makers that there is a practical reason for SOCAS.)