Here is a picture of Indians catcher Roberto Perez that was making the rounds on social media. I added the red circle to focus your attention to the topic of discussion.
Who me?
Notice the dark brown spot on the throwing side shin guard? What do you imagine that is? If you said “pine tar” you would be correct. Now … why do you suppose he put it there? If you said “to get pine tar on the ball for the pitcher,” give yourself a pat on the back.
If asked, a catcher will say that it is there to help him get a better grip of the ball but make no mistake about it. It primarily is to help the pitcher.
Pitchers and catchers both know that the guy standing on the bump will be the one monitored for any hint of illegal substances on the body/uniform. That’s why the catchers do it. Nobody is watching them. If pine tar appears on the ball, how many opposing teams will demand that the umpire check the catcher? Zero.
Of course, having too much of it like Perez can shift some unwanted attention the catcher’s way so most handle this with a little more discretion. Often a catcher will put pine tar in the pocket of his glove so it gets on every ball that is caught. It also is impossible to see. Others put it on the leather strings hanging from their mitt. Tugging on the strings is a natural movement for catchers behind the plate so doing so will not cause alarm.
And while I’m on the subject, I’ve even seen catchers sharpen the hooks on their shin guards (that are on the outer side of the leg of course) so that when the pitcher needs a little “extra something,” they just brush the shin guard with the ball during their normal catch and throw motion. This can put a little cut/scuff on the ball for their battery mate.
Before I finish, let me just say that explaining all this is not the equivalent of condoning it. My point is, if you think some “funny business” is being applied to some of the baseballs, you may want to focus more attention on the catcher.