For what reasons does Jem cry once he learns the verdict?

In Chapter 22 of Harper Lee's To Kill a
Mockingbird, Jem's face grows "streaked with angry tears" as he declares, "It ain't
right, Atticus." Prior to that moment, Jem grasps the balcony rail so hard his hands were white
and "his shoulders jerked" with each guilty verdict from the jury that Judge Taylor read. The
reason for Jem's reaction is he saw, just as Atticus saw, that the
evidence of the case clearly indicated Tom Robinson's
innocence, not guilt.

Jem begins to demonstrate how much he
understands the evidence is pointing to Ewell's own
guilt the moment Sheriff Tate testifies that Mayella had been bruised on the right
side of her face and had bruises circling her entire neck from a strangle hold. The right side of
her face could only have been bruised by a left-handed man facing her, and a man would need to be
able to use both of his hands to bruise her entire neck. Atticus and Robinson understand the
significance of Sheriff Tate's testimony, as evidenced by the fact that they exchange words. Jem
is apparently observant and old enough to also register the significance, as shown in the fact
that Tate's testimony makes Jem extremely happy.

Jem also demonstrates he understands the evidence points to Ewell's guilt
during Ewell's own testimony. Atticus cleverly asks Ewell to write
his name before the court to prove that Ewell is ambidextrous. Jem demonstrates his understanding
of the importance of this evidence when he pounds the rail in excitement and whispers, "We've got
him"; in saying "him," Jem means Ewell (Ch. 17).

The most telling moment in the
trial is when, during Mayella's testimony, Atticus has Robinson stand before the
court to reveal his shriveled left arm and hand, and Jem exclaims, "Scout, look!
Reverend, he's crippled!" (Ch. 18). Jem can very easily deduce that if Robinson's left arm is
crippled, he could not possibly have hit Mayella in the right side of her face nor strangled her
with both hands. Ewell was the only one present at the scene of the alleged crime that has
demonstrated the ability to use his left hand.

Since Jem sees clearly just how
much the evidence proves Robinson's innocence and points to Ewell's guilt, Jem is shocked beyond
belief to hear the verdict of the jury. Hence, Jem cries because he
knows an innocent man has just been convicted based on
racism, which is exactly what Jem means when he says, "It ain't right, Atticus"
(Ch. 22).