The problem is that Extra Spell does not override the RAW of the given class. For example, wizard was mentioned in this thread:

A wizard casts arcane spells which are drawn from the sorcerer/wizard spell list.
Extra Spell does not override this rule; You get a new spell, but it doesn't say anything that allows you to ignore this.\

Compare to the Psion:

Choose the powers known from the psion power list, or from the list of powers of your chosen discipline. You cannot choose powers from restricted discipline lists other than your own discipline list. You can choose powers from disciplines other than your own if they are not on a restricted discipline list. (Exception: The feats Expanded Knowledge and Epic Expanded Knowledge do allow a psion to learn powers from the lists of other disciplines or even other classes.) A psion can manifest any power that has a power point cost equal to or lower than his manifester level.
And even without the explanation given within the Psion entry itself, Expanded Knowledge provides this clarity on its own.