West Brom were heading for a first league win at Stamford Bridge since 1978 when Ramires went to ground in the box in stoppage-time after a coming-together with Steven Reid.
Eden Hazard scored from the penalty spot to earn Chelsea a 2-2 draw, but Marriner's decision was heavily criticised by West Brom boss Steve Clarke.
However, Riley has defended the way Marriner, who will take charge of Fulham's home league game against Swansea on Saturday, reached his decision to give Chelsea the spot-kick.
"I understand why Andre gave it," Riley said.
"He thinks he sees Ramires getting in front of Reid, catching his back leg and making him off-balance without playing the ball so it's a penalty."
Riley did not judge either way whether Ramires was guilty of diving, saying: "The truth is only Ramires truly knows."
Generally speaking Riley believes players in the English top flight have recognised the moral imperative to clamp down on diving.
Riley cited Premier League statistics to demonstrate that the practice of simulation is in decline, with six incidents identified so far this season compared to 19 by the same stage last term.
"We've seen a drop-off in simulation offences," said Riley, the general manager of Professional Game Match Officials Limited.
"We should be proud it's not an accepted part of our game. Players say it is not acceptable. Supporters said it is not acceptable.
"We mustn't lose the moral argument; it puts the onus on players not to do it."