Hello Yesooho91
The reason is a legal one.
If you purchase something downstairs for take away, the VAT is lower than in the restaurant itself. Therefore, people with take away goods cannot use the restaurant facilities otherwise the restaurant owner has to apply the normal VAT.
This is the same everywhere in our country, whether at Migros Restaurant, Mc Donalds, or in a coffee shop.
And yes, the table upstairs are usually kept free for people wishing to dine.
Kind regards,
Roland