The key word is business justification. Does the appstore no longer function if there's another store that also sells iOS apps? Sure that other store might become more popular and put the appstore out of business, but that's business. There is no justification why other companies aren't allowed to start their own iOS store.
Sure there is. Security, stability, network management, etc.
So, is cydia (and its apps/tweaks) legal or what then?
That's a two part question. My understanding is that jailbreaking and Cydia are both legal for the installation of legal and compatible apps on smartphones. (There is no exemption to the DMCA for tablets.)
My opinion based on copyright law is that most apps and tweaks on Cydia don't meet this requirement as they require modifications to iOS to provide unsupported features. But an App Store app distributed by its developer through Cydia would seem to be legal.