I’m pretty sure this is a felony. I’m not against the spirit but want to warn others that you could get slapped.
What exactly would the scammer/hacker say to the police/their lawyer? "Could you please help? I'm trying to scam innocent people out of their money, but some greyhat has filled up my server's logs with junk!"
So, crafting API calls to intentionally bring down a system most definitely can be a criminal offence.
Now if you're objection was to the use of felony as opposed to misdemeanor, then ignore whatever I said above.
The DoJ is unlikely to prosecute a vigilante of scammers, but they have prosecuted others (eg. Aaron Swartz) who were doing a debatably morally upright actions while violating the letter of the law.