Using comma to delimit the name of a group and its constituents?

Question

You are probably familiar with this comic which illustrates the possible ambiguity caused by leaving out the Oxford comma:

However, this strikes me as plain wrong. I thought comma would not be used to delimit the name of/word for a group and the listing of its constituents. The sentence given as an example on Wikipedia regarding the usage of the em dash indicates one would use either an em dash or a colon for this purpose:

Colon-like use

Simple equivalence (or near-equivalence) of colon and em dash

• Three alkali metals are the usual substituents: sodium, potassium, and lithium.
• Three alkali metals are the usual substituents—sodium, potassium, and lithium.

(Wikipedia)

So one would think that, in order to convey what is portrayed in the second picture in the comic, the sentence should be:

We invited the strippers: JFK and Stalin.

or

We invited the strippers—JFK and Stalin.

So, with all that said, my question is:

Is the sentence without the Oxford comma considered ambiguous because too many people are not familiar enough with the rules of punctuation of the English language, or are the rules laxer than I thought and it is valid to use a comma in this case as well?

(Bear in mind—English is not my native language, so the latter may well be the case.)

Answer 1

A comma is used to announce a non-restricting apposition:

My sister, Emily, is getting married next week. [I have one sister.]

but is omitted with a restricting one:

My sister Emily is getting married next week. [My sisters Joan and Anne aren't.]

If the apposition contains a list (with commas), then the first comma is in danger of being ambiguous (does it introduce an apposition or is the whole thing a list?).

My sisters, Emily, Joan, and Anne, were all at the concert [is that 3 ladies or n+3?]

...

My sisters, Barbara[,] and Amy[,] were at the concert [is that 2 ladies or n+2?]

With the latter sentence, if you say it with its intended meaning, you will hear a pause at each comma. If you leave out the 'Oxford' comma that precedes the coordinating conjunction, you may be judged to be taking a shortcut in orthography, but the pause is still required when Barbara and Amy are additional to your sisters. The shortcut is bad when near an apposition, although alternating orthography can clarify the meaning (dash, colon, etc.); but the pause in speech is still a short one.