John Mehaffey, who is no fan of WSOP.com by any stretch of the imagination, is actually on WSOP's side on this one:
https://twitter.com/twt/status/1220577604370984960
Another tweeter questioned why they would need to end the drawing early if they were planning to rig it in some way, which is actually a good point.
https://twitter.com/twt/status/1220597662849699840
My opinion: While secretly ending the promo two days early is very weird (especially because it's a drawing, and their exposure is already known), I don't believe it's illegal. Furthermore, as was pointed out, if the drawing was rigged in some way, the last thing they'd do would be end it early and arouse suspicion of something not being right.
I can't venture to guess why they ended it early, but they probably have a right to do so. I suppose a case could be made by Dewald that any play he made after the drawing should result in a rake refund for those hands, because he was playing with the expectation of earning promo tickets. However, as he found this out on January 20th (one day after they ended the promo early), I can't imagine we're talking about a lot of rake here. Additionally, he would still probably have a hard time getting Nevada Gaming to go along with this rake refund demand.