> It is kinda silly that ECC support is something you have to pay extra for,
As far as I know for Ryzen processors it is available in every processor.
> Why not just leave it out and use it as an up-sell for corporate users?
One trivial argument might be cost (indeed I consider it as plausible that it is cheaper to just produce one version of the chip).
But I also consider it as plausible that there is something deeper behind: Intel vPro (of which AMT is a part) is also used for ant theft (cf. [1]). Don't ask me about details how this is implemented - at least to my knowledge information about details are not completely trivial to find and Intel is rather secretive about anything that involves security. Now if only some Intel processors implemented Intel ME, Malory ([2]) could perhaps simply replace the processor by a one that does not implement Intel ME to circumvent the Anti Theft Technology (or at least this is something Intel fears). I consider it as rather plausible that something related to this might also be a reason why Intel does not sell processors without Intel ME (but consider this paragraph as speculative).
Wrong.