I don't know that why we have to pay those, who already earning their wages. Why we have to pay extra for what we have already paid? The staff is rendering its services on salary/wages basis which are paid through customers payments.
Because you HAVEN'T "already paid." You've paid a price the owner has set based on the assumption that you'd be tipping to supplement the sub minimum wage he's permitted by law to pay his employees.
I don't know why people have such a hard time understanding this simple concept. Eliminate tipping, require the restaurant owners to pay servers a fair wage, and they'll raise menu prices to cover the increased overhead. Then you'll all be whining about how expensive it is to eat out.