I don't quite understand the law in the 1st place, but what I gathered reading about this earlier is there are a bunch of German Catholics who don't want to pay the "voluntary" tax so they withdraw from being a registered Catholic so as not to have to pay but still attend services and want to be Catholic.
Gotta agree though, this is going to backfire... but IMO is only hastening the inevitable.
Joseph Smith got it right when he wrote his book (of course times had changed) by making tithing a requirement right from the get go if one wanted to be "saved". Catholic Church is playing catch up in this respect since paying for indulgences went out of vogue.