The Bath flanker had been on the pitch only four minutes, as a blood replacement for Richard Webster, when he was sent off by Steve Lander, an international referee, for the incident in a ruck.
Bath watched the video of the game and imposed a three-week ban on the 21-year-old, but he did play for Wales in their international against Romania the following week. The club, represented by Thomas Sheppard, a director, told the hearing in Bristol that they had asked the WRU not to play Thomas.
The hearing, chaired by Jonathan Dance, an RFU council member, took only 15 minutes to reach its decision. It took into account the club ban, so Thomas will be eligible for the League game at Leicester on 25 October.
"Nathan has behaved absolutely properly from the start, he accepted the punishment from the club without question and we felt we had imposed an appropriate penalty on him of three weeks," said Sheppard, adding that Bath might appeal.