You often see decades abbreviated this way — the 20's, the 50's, the 80's. But I believe that the proper use of the apostrophe in these cases is like this — the '20s, the '50s, the '80s. Because the apostrophe replaces the '19'. I'll bet it was the "straight A's" convention that led people to accept the abbreviation with the apostrophe between the 'zero' and the 's'.
That's a theory and it may well be a good one, hightor. Let's assume it is accurate. To suggest then that because language once did something one way and that is the proper way is [don't be hurt, for that isn't my intention] silly, fatuous, illogical.
There are thousands of changes that have occurred in language, all languages, and the only thing that is proper is what it is at the present time.
People have been raised by unthinking teachers/parents/... that there are certain correct ways to speak and write. Almost everyone could come up with a number of these bits of nonsense because everyone has been subjected to them.