Why should we consider something that happened 250 years prior as some sort of affirmative defense of the Crusades as having been something that started with the Islamic world being the aggressors?
If the US were to start invading Axis countries with WW2 being the justification we'd of course be the aggressors, and that was less than 100 years ago.
Because it played a role in forming the motivations of the Crusaders? It's not about justifying the Crusades, but understanding why they happened.
Similarly, it helps us understand all the examples of today of resentments and grudges over events that happened over a century ago that still motivate people politically.