SQL took a long winding road to reach the FOSS friendliness it has today. Its history is littered with patent, copyright, and trademark disputes. Not to mention widely varying implementations and the continuing lack of a crossplatform wire protocol or standard interface. Proprietary incumbents still dominate huge sectors of the SQL landscape and pose compatibility and intellectual property issues for FOSS implementations.

I think any SaaS APIs that reach any degree of widespread adoption eventually get OSS implementations (eg S3's API is widely supported by FOSS and proprietary implementations). This evolutionary path is awfully similar to SQL's for better and/or worse.