In germany there is something called "Reinheitsgebot" - the oldest (1516) food law in existence (if you don't count religious "laws"). It states that you can only use water, barley and hops for beer production. So no fish in german beer.
The Reinheitsgebot sounds like a good reason for brewers to be lawyerly about what counts as an "ingredient". So traditionally they used it but claimed it wasn't an ingredient, and then when mechanical filtration improved, they decided that was a safer course.
Since it's not an ingredient, the reinheitsgebot doesn't apply – none of the filter ends up in the final product. Hausenblase was used heavily for centuries, but nowadays it's simply too expensive to bother with.
It isn't a law anymore. But I do not know one beer brewery which doesn't still brew according to it. However, the food industry is usually quite creative in bending the rules...
