When people have moved to leaving candy out, perhaps kids thought that not having them out means you're not participating. Also, when many houses have left them out, for efficiency, it makes sense to only go to those houses.
I'm glad to hear that the sign helped. Shows that some kids aren't just about getting as much candy as possible but also having a bit of fun.
Tolerance of intolerance is not a path to freedom. Choosing to do nothing will only mean that rights and freedoms will be destroyed. Call this the lesser of two evils if you must.
If you let intolerance fester, it will grow and take control, then it will start hurting and squeezing others. It'd be foolish to sacrifice freedom in the name of freedom.