Because professional athletes use Twitter to communicate with their fans (somewhat true), follow hot girls (very true) and show off how interesting and well-rounded they are (rarely true), sports leagues have decided to use players' Twitter handles as a gross promotion vehicle. It's even invaded the teensiest of details: baseball batting gloves.
How useless is throwing a players' Twitter handle on batting gloves? Completely and utterly. First of all, batters are typically very far from TV cameras so no one will even see this? Second, maybe on replays or closeups we'll see the handle blink for a hot second but only people with eagle eye vision and DVR wizardry skills would be able to figure out the silly Twitter handles of baseball players while they move and like, try to hit the freaking ball. You might as well put the Twitter handles on the stitching of a baseball.
Perhaps, the only reason I can think of on why this exists is for the players to keep track of each other's Twitter handles while on the field. Imagine the conversation on first base.
First Baseman: Hey Justin, you just got a hit off my pitcher, he's terrible right?
Justin Upton: Ha, yeah. I'm going to lead off a few paces from the base if that's cool with you.
First Basemean: Totally, man. Oh, hey do you do Twitter?
Justin Upton: Yeah, man. Follow me at Jus-ten-up-ten
First Baseman: Huh? What was that again?
Justin Upton: Just-ten-you know what, here's my Twitter handle *hands over gloves*
First Baseman: Aw, thanks dude...
Justin Upton: *starts stealing second*
First Baseman: Are these o's or...ah, crap.