why does straight A's have an apostrophe?

I think it's only used so that it doesn't look like the word 'as'.

I agree that would be a good reason, but I'm not sure it's the reason, Rog. Seems like that's usually done when you want to pluralize a single letter, e.g., B's, C's, D's, etc.

Well yeah, but that almost sounds like we do it because we do it.

Exactly! That's the way it is with words, eh? Actually, it's probably useful to avoid confusing constructions like fs or rs. The convention seems to be to capitialize the single letter, insert the apostrophe, then end with the s.

Isn't this the explanation for every grammar rule in English?

Except that the 's usually indicates a contraction or a possessive. I think the OP was hoping for a little more.

just to be clear, only letter grades follow this rule?

No, I don't think it's limited to that. I might say, for example, "remove all the J's from that word."

letters, then?

People differ on the do's and don'ts of using an apostrophe for plural nouns.

Yeah, I think so. See my second post (I added to it, earlier).

greengrocer's apostrophe: An apostrophe used incorrectly to form the plural of a noun through ignorance of the use of the apostrophe.

I was under the impression the people who differ are just uninformed of the correct grammar.

EDIT: i think people tend to put apostrophes after plural nouns ending in vowel letters to make it easier to read/look better? i'm not entirely sure.

do's and don'ts

otherwise it makes no sense for you to put an apostrophe for do's and not don'ts to make it don't's

This is as straight as straight As get:

If ifs and ands were pots and pans there'd be no work for tinkers' hands.

Ignorance? Bullshit!

English has not been spoken with the correct "grammar" in the past 500 years. People keep speaking English in the way that makes sense to them, and eventually the "grammarians" catch up.

The grammarians used to say it was incorrect for you to boldly go forth, to have an agenda, to note what the data says, to find a free seat, or to have a fun time. This sentence would be frowned upon by grammarians of the past. But, once something makes enough sense to enough people to become common usage, it becomes correct.

English belongs to the "greengrocers".

http://www.quickanddirtytips.com/education/grammar/apostrophe-catastrophe-part-two?page=1

from Better Punctuation

The Exceptional Apostrophe
Single letters, numbers, symbols, and words used as terms look awkward when written as plurals without any punctuation, so we generally add an apostrophe s to them.

There are two a's in the word "separate."

His grades are all in the high 90's.

Can you change this 10 for two 5's?

He lived in the 1960's. I live in the 2000's.

(This answered my question about New Year's Eve.)

In one of my English classes at UGA, my professor followed my example using As and 1980s, and asked the class to lose those apostrophes.

💕It was love.

Good example. Even the most violent apostrophe haters have to write "do's." And the choice of "don'ts" is confounding too.