Why is there a Foursquare v2 API javascript authentication method?

Browsing though the Foursquare API documentation, I noticed that when obtaining an access token with ajax there is no need for a secret key (for obv开发者_JS百科ious reasons). But this makes me wonder what is even the point of having to register apps with foursquare in the first place if you can just fake being any app by using their client key. Why do they allow this?

https://developer.foursquare.com/docs/oauth.html

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Your answer is in the document you referred to

Since each credential is tied to a particular URL

The point is Foursquare is looking at the URL of the requesting script for security. If there is no registration there is no URL to tie the credentials to.