It's just that their imagery is intentionally quite jarring to christians. To quote the satanists. It is not them who believe in Satan, its the christians who believe in Satan.
All this may be quite dependent on where you live (the mention of the "church tax" makes me suppose Germany, as they have a Kirchensteuer, but I'm sure other countries must have a similar tax). In any case, there are European countries without church tax, beyond normal taxes that could be used for preservation of historical sites, religious or otherwise (which I agree may be somewhat biased in what religious sites can pretend to be considered historical).
Similarly, many countries do not have "religion" classes in "publicly funded schools", especially in countries that are (supposedly or admittedly) laic. That being said, there's also often a bias there as a lot of holidays are tied to religion and Christianity in particular, and it'd be quite common to explain in class the origin and nature of these holidays. I'd hardly think it counts as "religion" class, though, but that'd depend on what the teachers do.
Other european countries are more laical or secular, but that is pretty much just flavor today.
Some eastern countries are more involved, but if you propose the church having relevant influence in central or western Europe, you really slept the last two decades.
But, I still find this funny.
The problem is that Europe is split up into dozens of countries with very somewhat different values and cultures, so whenever you say "In Europe X happens", then you can always invoke some version of your argument.
Europe is France, Netherlands, Albania, Moldova. All of these are very different in various statistics.
But, how common is it really in Croatia or Poland to use christian terms such as "satanist" when you want to say that you are an atheist?
Seems anti-social, especially when injected into a non-religious context like a site about web design.