You could say that of killing anyone. There are good liches. A lich could be Lawful Good, love pursuing and gaining knowledge, and enjoy beind undead as much as anyone ever enjoys being alive--and would you still say killing the lich for being undead would be "not evil at all"?
Well i would be the first to admit that my knowledge of FR lore is limited so feel free to correct me if i am wrong but i believe that there is a big difference between the two.
A lich that has managed to remain good through the transformation WANTS to be a lich correct? He has done all the rituals to transform himself out of his own free will. He wants to remain a lich and is happy as he is. If the lich wanted release he could simply kill himself since this is something he is capable of. If this lich did not hurt anyone then i see no reason why he should not be allowed to remain in peace.
A revenant on the other hand is someone who has risen from the dead because his physical remains have been disturbed correct? He does not WANT to be an undead and wishes rest but cannot do so until he regains the item that has been stolen from him. In BG1 you encounter a thief who has grave robbed a dagger which he then proceeds to give to you, you can then enter the grave and give the dagger to the revenant and when you do he says something about being able to rest and be in peace now and collapses.
The difference here is obviously free will. The revenant is forced into undead form against his will. A protagonist that robs a grave might be responsible for an innocent soul being enslaved into undead form for who knows how long. It would be comparable to a powerfull necromancer somehow capturing the soul of an innocent and transforming it into undead.
In any case the presence of revenants proves that in some cases there remains a link between the possessions and the spirit of the deceased. In my view this makes grave robbing quite wrong since you are not only stealing from another being (the soul) but also risking that being returning from the dead in an effort to recover what has been stolen from him.
And finally lets not forget that even if the deceased did not care about his mortal posessions anymore grave robbing would still not be a victimless crime. Consider the wife of a soldier that was a good and kind person. Now consider that this soldier dies. It should be obvious that the wife who loved his husband dearly would be quite bereaved at his death. Now consider if this wife were to make sure that the very valuable wedding ring who was worn by the soldier remained in his hand while he was buried, something that gives the grief stricken wife some measure of comfort. Now consider the emotional and mental turmoil this poor wife would go through if she learned that someone had callously robbed this wedding ring from the hand of her husbands dead body? I would say that in this case there is most certainly a victim to grave robbing. The wife who is unable to grieve properly because of the actions of a cruel grave robbing adventurer.
This does not even consider the fact that according to law that valuable wedding ring might very well belong to the widow and as such taking it from the grave where the widow has stored it would be no different from stealing from a shopkeeper. Stealing may not be as big a crime as murder or creating a revenant but it still a crime, something that a paladin for instance should not be able to do.