This is to ensure data integrity and avoid mixing data across multiple stores. Let's say you have the following stores:
mysamplestore-1.commysamplestore-2.com
You need to use 2 different accounts to connect to these 2 stores, irregardless of the ecommerce platform (Shopify/BigCommerce/EasyStore) it is running on.
You will be prompt to switch account if you try to connect to a different ecommerce store.