If You Really Want to Get Technical About It...
The Droids are rightfully the property of Darth Vader, and he's just trying to reclaim his property.
Lets start from the beginning. R2 belonged to Padme, and C-3PO was built by Anakin. When Anakin left, his Mother kept him and brought him with her to the Lars Farm. Now it gets a little tricky. Since Shmi was still alive when Anakin arrived, 3PO would either be split 50/50 between Shmi and Cliegg, or still 100% Shmi's depending on if they had a prenup or not. If it was a split then Anakin could make a claim on sole ownership if Cliegg agreed, which he obviously did. In the novelization, Aunt Beru tells Padme that they can take 3PO with them, since Anakin was the one who built him. So Anakin and Padme entered their marriage each owning a droid, and now they co-own the pair, again assuming no prenup. Ironically, Anakin became very close to R2, since he accompanied Anakin and Obi-Wan on their missions, and C-3PO stayed to care for Padme and probably was useful when she was meeting with non-English speaking Senators.
Now for ROTS, R2 arrived on Mustafur with Anakin and was told to stay with his ship. 3PO arrives with Padme and stays on the ship. R2 makes his way to Padme's ship and they all leave with Obi-Wan, and Padme who dies shortly after. But Anakin is still alive. So the Droids would be his sole property.
Unless we want to use Obi-Wans "certain point of view" and say that Anakin Skywalker was dead, and Darth Vader was someone else. In which case they would belong to Luke/Leia.