Flamestrike
Legend
Where? Where does it say that other than the standard "the DM can change things if they want"?
Read the entry for Fiends and Angels (monster types).
Despite being literally comprised of cosmic good or evil and thus inherently good or evil by nature, both angels and fiends exist of evil and good alignments respectively.
Heck Grazzt used to be a LE Devil before he was a Demon. Zariel was a LG Solar before she was an Archduchess of Hell. Even Asmodeus himself is hinted at being a fallen Angel, as indeed are the entire species of devil the Erinyes - literally all of them were once (cosmically and inherently) LG Angels.
Or are you saying that only Angels and Fiends can be different alignments (and Drow because Drizzt, and Baenre whatshername) but Orcs... cant?
Like; the thing that makes the Drow 'evil' is they follow the edicts of Lolth and her evil pantheon (enslave, murder, betray). Those Drow that instead worship Elistraee (liberate, compassion, altruism, acceptance) are Good in alignment.
Ditto with Orcs. If you raise an Orc outside of its culture (and the worship of Gruumsh) and within the worship of a Good deity, by Good people, you likely wind up with a Good aligned Orc.