Because Tito was still playing to win at that point, which means trying to save your closer for a chance to actually close out a lead.
The original inning was Bartlett, Iwamura and Upton, two righties and a lefty not known for game-changing power. But when Carlos Pena came up, the formula changed. "Play to win" now involved neutralizing TB's best lefthanded hitter by any means necessary,because the alternative was an instant loss.
Again, if the Sox score in the top of the 10th, no one has a problem with what Tito did, and with his bullpen taxed, they weren't going to last much longer anyway.
The immortal Paul Byrd, Tito's only other option, surrendered 4 runs in relief in the very next game, so again, if the Sox don't score in the top of the 10th there really isn't much Tito can do to keep that game going. He got miracles out of that bullpen in that game as it was.
Tito's handling of Game 2 was tactically sound and followed conventional wisdom to the letter. Your problem is either with conventional wisdom itself, or with the fact that Tito's moves did not result in a win. Tito did the best you could reasonably expect with the roster he was given in that series, and with our injury-sapped rotation, Beckett reduced to a shadow of himself, Daisuke wild and no credible #4 starter we still got to game 7 with out best remaining pitcher on the mound and every chance to win. Pity we didn't, but there's enough blame to go round.
I'd be more inclined to focus said blame on Theo myself for not taking the chance to strengthen that roster when he could have, and for making moves that sapped our starting pitching depth without responding to the resulting problems until it was too late to get a quality replacement. Also for not taking the opportunity to explore our other youngsters, especially Bowden, to see if they might help solve a known problem in time to get them onto a playoff roster. It's not like Bowden would have been that much worse than Byrd!