The WhoIS we are talking about in the pricing page is the Gandi one. It is not available for every TLDs has some registries don't support it. Majors one does.
We do not earn money when you use the site. There is no subscription fee, no premium stuff. We take our cut when you buy a domain from Nuage and/or renew your domains. It is transparent, and just to make sure it is clear, we do buy our domains from Gandi which is accredited by ICANN.
API is on it's way but not ready yet. We are basically trying to create a standard for domain names. Also a lot ore is coming like instant landing pages, service integration ...
It's actually not a registrar, but a platform that aggregate registrars in a single UI. You basically sync your accounts from registrars (GoDaddy, Gandi, ...) and it let's you manage them from the same place.