Answer by Anirudh Acharya:
Ekalavya belonged to a tribe that paid obeisance to the Chedi and the Magadha kingdom, then controlled by Shishupala and Jarasandha. These kingdoms were not exactly great friends of the Hastinapur empire. Jarasandha wanted to become the emperor and wield control over Hastinapur and Drona was indebted to the royal family of Hastinapur.
He could not possibly live in the patronage of the Hastinapura empire and let his training and knowledge go into enemy hands. Stealing or no stealing he could not let Ekalavya walk away with his treasure trove of knowledge because he belonged to the enemies of Hastinapur.
IMO it had nothing to do with caste or any such thing, as is made out to be in recent times.