This is one of those things that should have been covered in the wonderful book, "Eats, Shoots and Leaves: A Zero Tolerance Approach to Punctuation."
Then again, the English author probably gave two hoots about baseball. So there's that.
Any-who, let's get something grammatically straight, baseball fans and lazy sportswriters and editors (and Mariners' announcers who do this constantly):
"RBI" means "run batted in." Or, should a player knock in more than one runner, "runs batted in."
Example: Todd Helton had 1 RBI today.
Example: Chase Utley had 4 RBI on the day.
"RBIs" reads and sounds like, "run batted ins" or "runs batted ins."
Either way, it just be crappy English.
Got it? Good.
*Find a punctuation error in this post? Fuck you for noticing.**