My Shul has a learning program where they do NOT pay, and the Rav at the Opening of Bein Hazmanim spoke about Torah Lishma, etc. etc.
However, it is not so shver to give a monetary incentive — most bochurim who come are bochurim who want to learn and would learn anyways; this just encourages that they all come to a central location with a Kol Torah rather than learning at home/random shul, and stops a lot of shmuzing that would otherwise inevitably happen where bochurim meet bein hazmanim.
Yes, it is not ideal for somebody to only learn for the money. But that is not the point of these programs. AND the Mishna says ????? ???? ??? ???? ??? ???? ????? ??? ???? ?? ????.