Horwood penalised Barry Fuller for his challenge on Moses Ashikodi. And League Two's top scorer Grant Holt slotted home his re-taken penalty before heading home Chris Humphrey's cross to rescue a point for the Shrews, who were 2-0 down after 51 minutes.
"I thought it was a very brave decision because you see those fouls happening all the time but there are not many referees brave enough to give them," Simpson said.
"I thought it was a foul and fair play to the referee for having the bottle to give it. I also thought it was a brave decision to make Holt take it again because of the players encroaching in the box.
"At 2-0 down it's easy to think we've got a problem but we kept believing and pushing. And in the end their keeper made some fantastic saves to stop us from getting through."