Beyond the obvious difference that Microsoft has a monopoly marketshare on PCs, and Apple has a minority marketshare of mobile devices, mobile devices are categorically different because they are always with you. This generates a whole different slew of concerns around privacy and apps being able to access your information with or without your consent.

By maintaining control over which apps can be sold to end users via the App Store, Apple is able to offer a layer of control over how these apps can access (or not) your personal data.

There's nothing ridiculous about wanting to maintain data privacy/security on mobile devices, and that being of a higher priority with mobile than with a PC.