R. Yosi says, this is absolute theft. What is allowed was only permitted is for the sake of Shalom.
Most commoners tithe properly.
Since the prohibition to make Chulin Tamei in Eretz Yisrael is only mid'Rabanan, we are more lenient. The end of the clause deals with Tum'ah of Chalah, which is mid'Oraisa. We may grind our food or deposit it by people that eat Shemitah produce or eat their produce in Tum'ah, but we may not grind the food or take deposits from such people. We may deposit Terumah by a Yisrael commoner, but not by a Kohen commoner, for he feels free to touch it. One who takes wheat to be ground by a Kusi or a commoner - it keeps its status regarding tithes and Shemitah, but not regarding Taharah.