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Sure, but you still end up with all of your devices seized for months and data dumped while you explain that you run an unprotected wifi router.

If your WiFi is entirely unprotected, depending on jurisdiction, you can be treated as an accessory which too can have serious consequences.

Surely they'd have to prove intent?

has this ever been tested in court?

Then you would have to prove that it's someone who used the service not you. The burden of proof is not on the accusor.