The number of actions to be taken to subscribe in these cases are way more than the one action you need to take in the solution Sharun has proposed so they would slower and more difficult. This method also quickly informs users as to which community they are subscribed to, which considering the number of duplicate communities is also very useful
Also voyager has set a precedent where you can quickly favourite unfavourite a community from the side tab by clicking on a star toggle/indicator so this design is not a completely new concept and is consistent with what voyager uses currently