Earlier quoted context omitted.
Why? Why should Lenovo let 99% of their users use sub-par drivers when they reinstall Windows?
I think the RAID setting can be the default. But they should provide the option to change if users want to install linux. Most linux users are BIOS friendly anyway.
Because Internet forums. That's why. Search for "I can't install Windows 10 on my Lenovo Whatsyourface" and you'll get "Change this super secret setting to AHCI". And who's going to be doing those searches? Everyone, but specially people less computer savvy. The ones that know enough are here discussing Lenovo's decision.
Again, just to clarify: I'm not trying to defend Lenovo. What they did was a bad business decision, but maybe it was the lesser of two evils. Maybe they should've kept Intel accountable on this one - but who can argue with the only serious CPU manufacturer for laptops?