We're upset because the full legal power of the government was used as a weapon against him almost as if he was mass murderer instead of his "crime" being just setting information that was technically free in the first place, actually free.
We're upset because instead of waiting pensively for what his sentence would be and protesting what would undoubtedly be unjust, all we can do is remember who he was and triumph his cause in the hope that this won't happen again.
He didn't deserve to feel this was his only way out and he had so much more to contribute based on his past accomplishments.
I guess the problem with us identifying with Bradley Manning in comparison is he didn't do things like contribute to RSS and Markdown. But the information he set free is just as important.