Why is there — and should there be — a comma following a state or country's name?

This Capital Community College document best describes my dilemma: “When both a city's name and that city's state or country's name are mentioned together, the state or country's name is treated as a parenthetical element.” That’s fine by me; it’s a habit of mine. But I occasionally find myself questioning that “rule.” For example, is the state or country — bookended by commas — a non-restrictive element?

In some cases, this reasoning would check out:

On average, temperatures in Salt Lake City, UT, reach 90 degrees 56 days a year.

There is of course only one Salt Lake City. But when we say something like this,

The tallest building in Portland, ME, is 16 stories tall.

the state element is no longer non-restrictive. If ME is removed from the sentence — as the commas suggest it may be — then most readers will assume the city to be Portland, OR. Other cities’ names come to mind: Springfield, Salem, Washington, etc.

Is the comma following a state or country’s name arbitrary inasmuch as it is applied to each and every state/country (with a few exceptions, each of which can be found in the above-linked document and is not relevant to this discussion)? In many writings — and I assume them to be written by people who do not obsess over the English language as I do — the comma in question is omitted. And this makes sense, especially in instances when the name of state is essential to identifying the city (see my second example).

Should I therefore consider using a comma, always and without discretion; sometimes, at my discretion; or never?

Note that the same is true of dates. For example,

July 4, 1776, is otherwise known as “Independence Day.”

By the same token,

July 4, 2016, is otherwise known as “Independence Day.”

Once you've decided to include the state, and place a comma between city and state, the reason for the comma after is fairly simple:

The tallest building in Portland, ME, is 16 stories tall.

In this sentence "ME" is parenthetical, and meaning of the entire sentence is reasonably clear.

The tallest building in Portland, ME is 16 stories tall.

In this sentence it appears that the tallest building in Portland is named "ME", and "ME" is 16 stories tall.

You could interpret the state or province in Springfield, Calif. and Springfield, N.L. as a non-restrictive phrase, as an elliptical or parenthetical way of specifying the Springfield you want*:

Springfield, [the Springfield in] New South Wales, is a suburb of Gosford.

Springfield, [by which I do not mean the town in Nebraska but the Mennonite camp in] Belize, has many fruit trees.

As you observe, the same principle could be applied to dates, thus July 4, 1776 specifies the 4th of July in 1776 as opposed to all of the other 4ths of July going back to the last 4th of Quintilis.

But not everyone interprets it in this way; the comma is arguably simply a separator. Had postal services adopted a hyphen or some other punctuation mark to serve that purpose, we would not expect a pairing by analogy to an appositive.

Springfield–Gauteng has a dry climate, unlike Springfield-Louisiana.

So, I surmise it comes down to a matter of style, like so many things.

* Washington, D.C. is a separate matter.