It is a bit of a bait-and-switch since no other CPU feature works this way and Intel never mentioned this "feature" in all their years of disclosures about SGX.
Isn't that how most extensions work? And microcode has been around for at least a decade now.
I know of no other CPU feature that requires authorization from Intel.
TXT requires an ACM, which is essentially a small signed BIOS subset. At least ACMs are freely downloadable from Intel, and they don't look into what you'd like to run under TXT.