You support justice because it is a fundamental right, because perpetuating injustice is intrinsically wrong, no matter who the beneficiary or the victim is. That’s just a basic ethical duty.
The oppressed do not “earn” justice by mounting a sufficiently effective campaign, by asking in an appropriately polite way, by being intrinsically better people, by demonstrating their nonviolence, by putting you at ease that they never have been (or never will be) oppressors themselves, by prioritizing your political process and sensibilities over their needs, or by being appropriately grateful for your support. The oppressed are owed justice because everybody is always owed justice.
Justice is not earned, it is a right. And acting justly is a duty, for everybody everywhere always. Full stop.
Yes, it is strategically wise for the oppressed to campaign effectively. Yes, it is easier to do the right thing when the other party seems likeable. But those have nothing to do with the fundamental rightness or wrongness of a situation.