As Joe Offer observes this is "a long-time liberal cause" not a recent 'bandwagon'.

Unlike the OP and some others, the information available outside the 'normal cycle of appeals' fails to convince me of Peltier's
innocence and an undeniably flawed trial doesn't necessarily equate with lack of guilt either any more than exercising critical thought, sceptism about professional public relations and lobbyists campaigns and entertaining reasonable doubt makes me a 'right winger'.

Perhaps the most damning testimony I've read was that of former AIM activist KaMook Nicols then wife of AIM leader Dennis Banks.

During the trial of Arlo Looking Cloud for the execution style murder of Annie Mae Pictou-Aquash (considered the highest-ranking woman in AIM who was close to Leonard Peltier but in the increasing paranoia within the organization suspected of working for the FBI) Nicols claimed that Peltier had bragged to her and Annie Mae Pictou-Aquash about shooting both agents at close range although chillingly, he claimed one was begging for his life.
trial transcript

Although Nicols has been accused of accepting money from the FBI for her testimony Paul DeMain has also reported that he had met with several people who said they had heard Leonard Peltier in 1975 confess to the shootings of the two FBI agents at the Pine Ridge Reservation and that
"As editor of News From Indian Country, I stand by our credible and trusted sources, and my present belief, that the primary motive for the murder of Annie Mae Pictou-Aquash by other members of the American Indian Movement in Mid-December 1975, allegedly was her knowledge that Leonard Peltier had shot the two agents, as he was convicted."

I can find no argument with eanjay's contention that "irrespective of whether he is innocent or guilty people should be up in arms about the treatment he receives"

However, this should be applied equally to the estimated 20,000 plus prisoners held in isolation (which has become a "regular part of the rhythm of prison life") in the United States at any one time, and not just one, very publicized, case

If the OP was to promote a petition against the unnecessary uses of solitary confinement for minor infractions and to demand better medical and mental health care* for all then I would certainly sign.

* "There are facilities with four or five thousand people that only have two or three doctors." Around the country, some physicians are operating on a license that restricts their work to correctional facilities
because they are deemed not qualified to provide care in the community. misguided federal law deprives correctional systems of desperately needed Medicaid and Medicare dollars to fund decent health care. Many people in prison and jail qualify for these federal benefits and lose them when they are incarcerated. Just like any other community healthcare provider, correctional agencies should be reimbursed for the cost of providing health services to people who are Medicaid and Medicare eligible"
Dr. Joe Goldenson
Confronting Confinement report

Stop Prison Abuse petition

National Religious Campaign Against Torture. "Torture is a Moral Issue" Statemen