To be clear, you can replace the battery with whatever battery you want, it will just show a message saying it can't be verified as an Apple battery. The only user impact is being unable to see battery health. I don't even think I can see my battery health on my Android so I don't see it as a big deal, but definitely not great if they are hiding this functionality for no reason.
If their battery health tightly calibrated to report on Apple’s OEM battery, it could provide misleading information about third-party batteries.
My guess is that Apple is bored of dealing with complaints about devices which have been modified by third parties.