The marchers are trying to force Israel government to completely empty its prisons of Muslims in exchange for the bodies and a few living hostages.
The #1 terrorist raid organizer Haniyeh was one of over a thousand terrorists released in exchange for Gilad Shalit in 2011. That happened after Israeli surgeons removed what would have been a fatal brain tumor from Haniyeh.
Sinwar was also released. This is the reason hamas took hostages. They know that they can win if they get a ‘give them what they want, no matter what’ response. It’s worked in the past and they calculate it will again. No doubt there is U.S agitation for this very thing. As said, if Israel goes down this path, it won’t survive.