[tankm0de]

In end-to-end encryption the private keys are on the user's phone. The point of a wiretap is not to let the target know you're listening. Having the code source + user's public keys from WhatsApp is of no help in decrypting.

[simoncion]

Yup. E2E crypto with keys stored on the conversing devices shuts down all MitM attacks. It doesn't stop targeted attacks (warranted or unwarranted), but I expect that law enforcement considers passive data scraping to be much less serious than targeted surveillance.