Well, I kind of think it is more of speaking out about their own 'lack of belief' and why, which does make it an expression of opinion in contrast to the tacit support of religion which is what the nativity scene amounts to.
Thing is, a lot of people think that its only atheists who object to prayer or religious monuments/symbols of any kind in schools or in other civic venues. Actually, some of the lawsuits against prayer especially in schools were started by Christian churchgoing families whose teens were being bullied by other Christian churchgoing teens about how they were going to burn in hell etc if they didnt get born again. It was Christian on Christian bullying! The fact is, not all Christian faiths take that quite as literally as others do, and when other not christian faiths get involved, it gets even more complicated.
Now, Im sure that most parents dont tell their teens to browbeat other teens that way, though I have also seen adults do similar thngs to each other, but by putting a stop to all such things on public property, it avoids even the APPEARANCE of government support of any one religion or sect, and any sort of problems like that. People can decorate their homes, read and teach any religious info they choose, visit any church, temple, chapel, synagogue, shrine, oak grove or anyplace else they wish to anytime they want to
but by having anything at all on taxpayers areas, it gets complicated. I dont care if theres a Menorah, a Crèche, a Minaret, a statue of Buddha, and another one of Vishnu next to a Goddess symbol and that SAME sign saying everyone else is full of hooey. It doesnt bother me a bit, but if it bothers anyone at all, I say put them on private property and be done with it, dont ask taxpayers to support any of it.