The answer to both questions is... it's up to Apple. When you design and build a product it'd be up to you how you design for your revenue.

If Apple allowed side-loading apps or custom app stores that'd cost them more to build and support that functionality, and it would damage their existing functionality through extra complexity and security surface, harming their existing happy customers. They don't want to do it. Why should they? Use Android if you have a problem with it.