Quote Originally Posted by Maciamo
a basis for belief, action, or argument <ground for complaint> -- often used in plural b (1) : a fundamental logical condition (2) : a basic metaphysical cause

The "ground(s)" is the basis for the argument, but not the argument itself. For example, if someone says that death penalty should not be legal because "it is immoral" (the argument), I will ask them on which grounds they find it immoral. The answer could be "as a Christian, I consider that killing is immoral" (the part in green is the ground), which is different from the argument itself in blue)
But if you look at the definitions of argument you will see (among others).

A course of reasoning aimed at demonstrating truth or falsehood: presented a careful argument for extraterrestrial life.

A fact or statement put forth as proof or evidence; a reason: The current low mortgage rates are an argument for buying a house now.

You are only considering 'argument' as the second of those while the first can have several 'grounds for the argument'. It is clear from context and example sentences that the Japanese 論拠 is based on the first definition of argument and each 論拠 is what you would call an argument.

Anyway if you really want to know how to say something in Japanese why are you nitpicking over the English?