What does it mean to steal a model? It means someone (the thief, presumably) can re-create the predictions of the model without having access to the algorithm itself, or the training data. Sound far-fetched? It isn't. If that person can ask for predictions from the model, and he (or she) asks just the right questions, the model can be reverse-engineered right out from under you.

Relevant links:

https://www.usenix.org/system/files/conference/usenixsecurity16/sec16_paper_tramer.pdf