They haven't, but we're not talking about now. We're talking about later when this mess dies down. The hostages are likely to make a difference in Gaza's post-war fate. And given the stakes (Israel has been pretty clear they want to re"settle" Gaza) I'd say while the hostages are victims and deserve better we can't blame Hamas for holding onto them.
Edit: The hostages are also likely to be involved in things like how much food and other goods Gazans are allowed through the blockade, trying to get Israel to not do random airstrikes and other such things.