And I pay $99 a year to licence it.Unless you have signed an agreement with Walmart to give it a cut of sales of your property, regardless of channel through which the property is sold, Walmart has no right to a cut of sales.
Before you can use Apple's technology, you must sign a license which gives you limited rights to use and distribute their property. Your license is binding whether you sell their property through the Apple App Store or another App Store.
But Apples cut from another store or the open web should be 0%