It's not really something that can be proven or disproven by data convincingly most of the time, even in pretty extreme cases that could be measured (like Palestinians, which are genuinely oppressed by any objective measure) you can quite successfully rationalize the injustice away with good publicity machine ("that's because they're terrorists" or something).

I think a lot of it has to do with just how closely linked powerlessness and rightness are in people's minds. If someone supporting Palestine tells someone supporting Israel that the Palestinians are oppressed, the Israel supporter feels like letting this statement go would be equivalent to admitting that Palestinians are in the right and Israel is in the wrong, or even that Israel doesn't have a right to exist.

The best way to deal with this IMHO would be to say "Yes, the Palestinians are oppressed, but even so Israel still deserves its land." But this is a very difficult statement to make. The natural human response is "No, the Palestinians aren't oppressed!"