The European Union (among other things) requires that you also be able to obtain applications through other channels. The word “side loading” is not used. This is an information technology term.

I also have a work app store on my work phone. It contains (partly) home-made applications. This works through “Profiles” (as iOS calls them).
Thus the business owner can sign a home-made application and place it in his own store. (My work phone is also managed by my employer)

Now there will be a “public”/”public” version of this.

You should be able to get those to other stores somehow. So they will likely be in the App Store. I don’t expect the EU to think it’s a good idea to let people download/install a profile that allows them to install a separate package that acts as an additional app store and go from there. That’s a lot of trouble. So, you will always have a slight dependence on the Apple App Store. I don’t think anyone has a problem with that either.

It’s not crazy for Apple to require a certain form of control (quality/safety). (The fact that Google doesn’t do this with alternative stores is irrelevant)

Assuming that Efteling is also obliged to allow third party shops/restaurants, it is not crazy for Efteling to impose certain requirements (quality). You can go anywhere and nowhere to Efteling owned stores/restaurants/etc. Buying things, but also from that third party. As Efteling outlines the requirements: “The store must have a fantasy theme/look and must appear ‘neat and tidy.’ You may not sell liquor, sexual items, white goods, etc.”

That’s not unreasonable, is it? So it’s not unreasonable for Efteling to check this (in advance).