There is no specific law, or legal precedent disallowing a platform owner from exclusively controlling software distribution for their platform. In fact it’s been going on for a lot longer than the App Store. Several early smartphones before the iPhone were locked down in similar ways. Games console manufacturers and media player vendors often maintained exclusive services software and content distribution right. If n fact since you mention Microsoft, don’t they have an exclusive online store for XBox games? This sort of arrangement has been common practice for decades and never been legally barred that I’m aware of.