It's MIT licensed. If they're actually reselling the API (and not hosting their own instance) then it could potentially be in violation of the terms of service, but that's it.

Using images from Faker's CDN is scummy and potentially illegal, but fairly minor overall.

If you are using images that someone else createdbwithout permission or license that is copyright infringement. If you are also using data created from a service and use it in your product again without permission or license this is also copyright infringement.