Been there, and I didn't even realised the cause. I'm sorry if my previous comment seemed light hearted, I didn't want it to be so.
A device that you buy with your own money cannot "hand-wave" the contract (TOS, EULA, Policies, etc), and say it's OK that you don't own your device or your data.
It's somewhat surprising that the other low-end SoCs are locked by default... I wonder if the key is the same for all of them, and just hasn't been leaked yet.
The key is most certainly not the same, because I doubt they would go through the trouble of doing actual secure boot verification, and storing the data in the TEE, and just have the same key. Additionally, the U-boot code I obtained lies to the user about commands not being found, if the command doesn't contain a valid unlock key.
SC9863A and many other SOCs are flashable with Spreadtrum's ResearchDownload Tool. However, Spreadtrum actually does verify the whole boot process, meaning that booting a modified binary is impossible. If you change the boot partition, it will infinitely reboot with a black screen and vibration. If you leave the boot as-is, but change system, it will get to the splash screen and then reboot. etc.
It genuinely does cryptographicaly verify the signature and hash of every partition. Which is great for security, in theory, unless the OS has preloaded spyware, but the secureboot process prevents you from removing it.
re-edit: Doogee N10 costs 85$. I don't think you can go much lower-end, without basically giving the manufacturer a huge profit margin (i.e. a phone that costs $60, but has 512 MB RAM, has a bigger profit margin than a phone that costs 85$ but has 3 GB RAM.)