1. Contact -> List and Segments button -> Add to Subscription Lists
Go to Subscription List and observe Contact does not get added.
Go to Marketing Lists and observe Contact are added.

Hello,
The usecase for subscription lists is to be added on a marketing form and exposed to users so that they could opt in and out themselves. You shouldn't be adding contacts manually to them anyway.
Adding contacts to a marketing list manually is fine.
Thanks,
-DD