It has a public API, no registration required, accepts up to 10 checks per minute. A WordPress plugin and a simple PHP library are also included.
Check it out :-)
Sure if it's some throw away service that doesn't have such sensitive information, that's fine, but there's many reasons to use such a service.
The user's fault; they are knowingly trading that risk for protection of their privacy. That says a lot for how they regard the service in question.
What about rejecting ISP-issued e-mail addresses? Those are also ephemeral, for those who use them.
What about Yahoo! / Outlook.com / Gmail addresses? in the vast majority of cases people using those have no 'hold' on them and their accounts can be suspended at whim.
Domain-related addresses can be lost if the registry decides to hike the domain prices beyond affordability, such as with the 1000% increase in Uniregistry gTLDs later this year.
Where to draw the line? All e-mail addresses are temporary.