Well, that's pretty much what "providing SDK to vendors to customize their devices" means; if you're not planning to buy SOCs to make large quantities of custom devices, you're not a vendor for whom the manufacturer cares to provide an SDK.
Allwinner isn't the only vendor guilty of that. Ever try finding a datasheet for any Broadcom or Qualcomm part? I'm not defending the practice. I wish all SOC vendors would give full access to any/all documentation. They can still have a policy of "no support unless you sign a contract to buy X number of parts."
