The old folks only do it because they know the game better than the people that don't clap for the opposing team.
sarcasm aside, this statement is probably substantially a fact. us old farts grew up on baseball, played it, umped it, read the rule books, and witnesses many games at many different levels. with this vast experience comes at least a little wisdom and therefore an appreciation for a great play no matter who makes it. baseballs skill set allows this more than football where the play was successful because of about 5 or 6 other guys in the play while in baseball it the player is on an island.