The non-restrictive clause is the non-essential information added to a sentence after a noun or subject. It provides more description that may not be needed as the information will still make sense without it. In the sentence above, the clause, "which is free for all full-time faculty" provides more information about the college cafeteria but it is not essential as we will still know the additional benefits of the college cafeteria without it. They are usually separated by commas.
True, this was a quote that was very popular when slaves were around still. Symbolism doesn't always have to describe, usually just gives you a vivid image. I hope this was helpful
