The lowest common denominator of SaaS APIs is HTTP. Apart from that, everyone does it different.

Some use JSON, some use XML, some use SOAP... Sometimes you authenticate with Basic authentication, sometimes you use OAuth 1.0 and need to sign each request, sometimes it's OAuth 2.0 and you just need the token. Oh, and the actual GET parameter for the token is different, too...

There are APIs where you need to set a 'X-API-Version' header to something, or you'll get weird results. The list goes on...

The goal here is to hide all that crap from developers and focus on achieving the desired result.