Come on now. This really isn't a high bar and it only applies to the top tournaments. They will allow any legwear that is not jeans. Literally every professional sports has some form of equipment requirement and chess is probably one of the cheapest.
If you think world championship contenders can't be expected to afford normal trousers, what do you think athletes spend on running shoes, baseball gloves and ice skates?
A player broke the rules, which have been consistently applied for a long time, and refused to change after his first warning. Do you expect FIDE to make an exception just because he's the top contender?