Sure, they were turned away in
Buckland when the horn cry went up, but here's a quote from the book:
"
But the Black Riders rode like a gale to the North-gate. Let the little people blow! Sauron would deal with them later. Meanwhile they had another errand: they knew now that the house was empty and the Ring had gone. They rode down the guards at the gate and vanished from the Shire."
It is obvious that they were not cowardly, but meerly didn't want to deal with every
hobbit in Buckland at that moment. Why waste their time? That was not their purpose. Their goal was to take the Ring, give it back to Sauron, and then he could do the Bucklanders as he wished. The Ring was no longer there, so they left to retrieve it. Does that sound so cowardly?
As for
Weathertop, the Riders were not expecting to have any problem in taking the ring. They wanted it to be swift and ruthless, and did not expect any opposition. Why risk anything? Why not wait until all nine of their companions were there and avoid any risk of harm?
Perhaps they are cowardly, in their own right. But they are (or were) powerful, and besides, if
Aragorn didn't scare them all off of Weathertop, the story would have ended there. That wouldn't have been too great, now would it?
