There is no "standard". There is only what the market will bear. 30% seems to be an amount just barely low enough to avoid the attention of regulators in an environment just now absurdly forgiving of monopolists.
But, yes, Big Evil takes more than a walled garden.
Not in the technical meaning of the word, but for some reason, (app delivery) platforms (and software retailers) do seem to gravitate to a 30% cut and/or to treat it as some "baseline" ?
But, yes, Big Evil takes more than a walled garden.