The Internet, with its myriad of acronyms, can be a confusing place. You’ve got your WSDLs and your H2s and your WYSIWYGs. So it’s no surprise that people have started to add apostrophes where they don’t belong.
Case in point:
However, my guiding principle on whether to add an apostrophe has always been possession. Does the API own something, or is it simply a plural?
Confusion arises because the rules (according to AP Style) are different between single letters and multiple letters. To quote the Plurals section of the Associated Press Stylebook [2001 Edition]:
SINGLE LETTERS: Use ’s:
Mind your p’s and q’s. He learned the three R’s and brought home a report card with four A’s and two B’s. The Oakland A’s won the pennant.
MULTIPLE LETTERS: Add s:
She knows her ABCs. I gave him five IOUs. Four VIPs were there.
And from the same stylebook’s Punctuation Guide listing for Apostrophes: “DO NOT USE: For plurals of numerals or multiple-letter combinations.”
So people may have learned the single-letter rule once upon a time, and incorrectly apply it to the multiple-letter situation. (Or they may just think that “APIs” looks wrong and try to add some clarity.)