Why is this API being abused? Because it provides valuable information—which took a significant amount of effort to curate—about an email address.
The list of services which have lost my (hashed or not) password at some point ever in the past eventually turns into a list of every service I’ve ever subscribed to.
Whether or not it’s possible to scrape that information together, is it really something that should be available to pull over an API for a million emails a month?
Note this is very different information than the password breach count, which gives you an approximate count of how many times a given password has been breached, and works as a proxy for password strength without disclosing any PII.