It's completely legal to ignore a DMCA notice. These notices set up a procedure where you're definitely not liable, but it doesn't mean it's the only way to not be liable. You can also not be liable if you - for example - didn't do anything illegal.
What od you mean it's not legal?
If I don't live in the US and/or didn't do whatever the fraudulent DMCA notice accused me of doing, why do I have to process it?