I think part of the reason these people might feel this way is because when you look at the fundamentals, Israel was a really really stupid idea. The premise that one group has a right to some land because their ancestors lived there 2000 years ago is quite laughable. That's why I've always considered Zionism as absurd as the drivel written by Ayn Rand and about equal in what value it brings to public discourse.
And it doesn't help that Israel was founded by some pretty disgusting people, can anyone explain what makes the Irgun and Lehi in any way better than Hamas?
The key argument is that Jews need to have their own country, regardless of the location, because this is the only chance they have to finally be safe from persecution and be regarded as a people equal to others. Considering what happened, that is a quite compelling argument. Now, we can keep on rambling about the fact there were millions of other places to go, but that's pointless. They decided to settle there (which obviously makes far more sense than any other potential choice), they did settle there, and since they now live there, they have all the rights to have their own country. Which is the exact same reason why Palestinians have the right to their own country.
And reducing the Zionist movement to Irgun is like reducing socialism to gulags. It's absolutely ludicrous.