Both withdrawals were unilateral and, in the sense that the areas withdrawn from do not agree that they were complete, disputed. The only real difference is that Lebanon is officially a sovereign state whereas Gaza is only part of what might be one sometime in the future, still dependent on the neighbour it hates most. According to your argument Israel's actions in Lebanon and its unforeseen consequences (Hizbullah's very existence to name just one) are not its responsibility. Ruining someone else's back yard because their weeds threaten yours does not absolve you from responsibility for the damage you cause.
Israeli teen lightly wounded from stone throwing in West Bank settlement of Kiryat Arba (Haaretz)
from the article: Of kidnapping and weeping