Certainly it can be done, but it also adds a whole other process the tracks would have to deal with including selling the card, filling out the card and tracking or not tracking what the rider is doing. Are their points and results being kept or not, if not that impacts the scoring of the race and potentially the race results. It is nice to know when you get to the starting line who you are racing against. Starting place for the second moto, etc., etc. It can all be done, but it adds to the complexity of race days that are already long enough.
But the real issue here seemed to be the money amount, which in a sport where you are likely spending thousands a year to do seems like just looking for a freebie when everyone else has paid for their CRA card for the year. To me it is just part of the cost to play, so pay it or don't and go someplace else to race.