The rangers are highly secretive, so much so that the people of Bree and the Shire don't even know they defend them. Of course they let Elves and Dwarves travel the road, why would they want to stop them?

Saruman's enthral to Sauron is relatively recent, and he never took Hobbits in any way seriously, he has no direct control of Mordor's forces anyway. Frankly they dodged a bullet a bit on that one, Saruman's betrayal could have been far more damaging if he'd been even slightly less arrogant and dismissive of others.