It’s a sanction, designed to put pressure on citizens to influence their government. And if a Russian citizen is showing on a Russian horse at WEF and they know and are ignoring it- that is not a good look for the sport at all. And could lead to sanctions of the US team by FEI. Very stupid, if true.
ETA: I looked at the USEF online records and they do not show her competing in March at all so maybe this isn’t true. But the Plaid Horse lists her as coming in 10th place on the 6th in the 4* GP. How is she competing and it does not show up in the USEF search?