Not necessarily as it is on public land. I think it would be different if the religious symbol was used in some secular connotation or if it represented a single individual. Instead this memorial makes a kind of establishment of religion, by attempting to symbolize an entire nation's involvement in a war with a symbol from a particular religion. Thus, this or the 10 Commandments outside a courthouse is unconstitutional, but no one's contending the 'little crosses' on headstones in Arlington National Cemetery.

Being atheist merely means you don't have a belief in god. There are many 'atheists' that believe in the afterlife.

But alluding to 'scientific atheists' or 'secular atheists' (to which I subscribe), I think saying we 'have no hope' in an afterlife is kind of misleading. It presupposes in a way that eternal life is better than finite life on Earth. Most atheists I know don't have 'hope' in an afterlife inasmuch as they 'don't have hope' in a life before this one. I didn't mind not existing before I was born, and I have a feeling that I won't be around to care once I'm dead.

That said, I think you can be artistic (perhaps even far more meaningful) in the creation of memorials in a secular context, rather than having to find refuge in the supernatural or hope in the afterlife.