No one objects to the temporary roadside shrines. Not even atheists. That isn't what any of this is about.
What it's about is Christians persistently trying to have their religious symbols used as public monuments, and non-Christians objecting to it. And the fact is that it's not legal. Public monuments are built using public money, on public land, and require public funds to maintain. And we can't make our fellow citizens pay for religious symbols that they don't want representing them. Nor can we allow the government to present itself as being supportive of any religious dogmas, symbols, images, or moral codes.
But some Christians just can't seem to grasp this concept. They refuse to respect the rights of others not to have to pay for or be represented by Christian religious symbols, nor subjected to Christian religious ideology or moral codes. So they are continually trying to push these on everyone else.
I'm a Christian, and even I object to that.
This doesn't really have anything to do with the spontaneous expressions of grief that cause people to create these temporary roadside shrines. And that's why no one is particularly offended by them even when they include religious symbols. Though they become just more roadside trash after enough time passes, and then they need to be cleaned up, by someone, and that's is a bit of an issue, although minor.