From my very first post here, I have opined that these guys have every right go do what they are doing, but that they are just being dicks.
If I understand your position, it is that:
The majority of Christians put up their decorations to show their joy and sincerity in their beliefs, and not to be exclusive of, or put down other religions.
The Satanists, on the other hand, have made up a set of beliefs and imagery out of whole cloth. These beliefs are not designed to represent their actual beliefs, but specifically to give the finger to Christians.
And although you have no problem with their right to do so and would actually defend that right, you think they are actually hurting their cause by being dicks by purposely taking a dump in the punchbowl, as it were.
If that's a fair representation of your position, then I largely agree.
All except the part about them hurting their own cause. I think that some people will certainly feel that way, but perhaps not all. And I think that's the reason to do it, and to do it this situation.
Christians (some at least) have been taking advantage of being the dominate belief in this country. Deliberately or not they have been trampling on the rights of minority beliefs to a greater or lessor extant since the beginning. They've assumed they always had the right to use public property for their religious purposes but have fought against others. They've assumed their holidays should be the government's holidays and have fought against others. They've assumed their prayers should be used in government meetings and fought against others. And so on. This may be better in recent years (I believe it is) but it does still go on.
The Satanists are trying to point out this continuing suppression of minority rights. And there is no "good" time to do it. When was the good time to sit at the front of the bus? When was the good time to sue for marriage equality? When was the good time to march and protest for equality in housing, jobs, and education?
You do it when you can get some eyeballs on it and then hope you get at least some people thinking. And that will always be a bad time in someone's eyes. But hopefully not all.