Researchers agree these terms because they have no other choice. Ok, seems nobody outside the academic gets the sense of publish or perish.
Let me tell you why I continue to send my works to Elsevier (or the others) journals, whatever they are asking in the terms and conditions.
In my country (France), to get a research position you first have to get a "qualification" which involves a threshold on the number of journal papers you have. The higher the impact factor of the journal, the better it counts. Once you have this "qualification", you can try to get a position - the system is competition based, and most of the time it is based on the number of high impact factor journal papers you have. So yeah, basically, if you try to play the cowboy before you have the position, you'll never get one.
Now, I do have such position and I could put all my stuff on arxiv. But I also have PhD students, and they want to work in the academic. if I tell them to go the open access way, they'll never get the "qualification" and the position. Thus, we chase these "important" journals (read significant impact factor), and send the articles there. As long as articles in these journals is mandatory to get a position, we have no other choice than publishing there for the students.
To my mind, the solution lies not in the hands of the researchers, by is rather a political one. If the government dictates specific recommendations that positions should be awarded to people with open bibliography, the stupid behavior of Elsevier will die. As long as no political action is taken, it will continue as it was.