In the case of tortious interference, Google would be selectively attempting to interfere with the existing business relationship between the ISP and its customers by targeting specific ISPs it deems as violating what it considers as net neutrality.

Where as an ISP is unaware of any specific relationship and therefore isn't attempting to interfere.

IANAL but that's my interpretation of this specific tort law.

https://en.wikipedia.org/wiki/Tortious_interference#Elements