Amazon's not in the business of terminating accounts the second a copyright request comes in. They want to build trust with their customers that they won't just turn you off on a whim.
I can vouch for this, from the other side. We routinely catch AWS hosts running password-guessing bots against our login forms. Emailing abuse@amazonaws.com doesn't seem to lead to reductions in our fail2ban and custom tarpit logs.
Claiming someone is brute forcing your logins doesn't have the legal weight of a DMCA notice. Why should they do anything? DMCA provides provisions for counter notice and legal remedies for false filings.
This is always a battle - for big operations you've got people farming out signups using stolen data to random 'buddies' on the other side of the world with the dark hat team ready to stand up outbounding traffic as fast as they can get a processor to execute it on, not to mention the hosts that get cracked automatically..
That's their official response, and rightly so it should be. Talking from experience, I've had many requests for different websites I run and almost all were frivolous. Some content I took down anyways, and some I explained to Amazon why it was frivolous. They've never cancelled any of my services or even threatened to.