It can be a crime if it is a violation of the site's terms of service and the requests violate the host's robots.txt.
I'm not sure whether there is a legal precedent though, in some cases you could call it a denial of service if the requests are not rate limited, and in other cases it might be considered an inappropriate access (see Weev, though he eventually won appeal).
Actually, Terms of Service violations fall under the Computer Fraud and Abuse Act, since ToS agreements can lay out under which circumstances that authorization for access to computer systems is given. That sort of obscene generality is the reason for proposals such as Aaron's Law, but to my knowledge there are no such protections today.
I'm not sure whether there is a legal precedent though, in some cases you could call it a denial of service if the requests are not rate limited, and in other cases it might be considered an inappropriate access (see Weev, though he eventually won appeal).