You believing all property is theft is very avant-garde of you, but at the same time it is not a stance the vast majority of the world agrees with (including Germany), so it hardly seems relevant to a constructive conversation centered around the behavior of German ISPs.

It's not avant-garde, the expression in this form for what I know was coined by the 19th century German philosopher Max Stirner.

The nub of the issue though is not really if something is theft on a legal definitional level. Laws themselves are extremely rarely enacted by direct decisions of those who are commanded to follow them. so they don't reflect what the vast majority of people would consider moral, which often include reciprocity, fairness, and staying beneficial to the society as a whole rather than benefit a tiny minority with highly detrimental consequences for the rest of people.