why was there no point to bind the dwarves

There was no point in binding the dwarves because they were exhausted, but the main reason would be the magic doors.

The prisoners were brought before him; and though he looked grimly at them, he told his men to unbind them, for they were ragged and weary. "Besides they need no ropes in here," said he. "There is no escape from my magic doors for those who are once brought inside."