Case in point: if you develop for A9, you will see many listed as OEMs for that chip but you will hardly find any datasheet or reference manual for those chips without signing away your family.
And the development there is done pretty much exclusively with OEM-provided sdk, much in the same way as Netduino is doing.
The general trend makes sense as dealing with hardware is expensive: for small micros, you code in assembly; for medium micros, you code in C + libraries sometimes; for big micros, you code via sdk / os.