Baseball double-header economics are different. Some of them are planned in advance. For the unplanned ones, the season has so many games that the team knows there will be a few unscheduled ones. Also, when a fan buys a ticket, he's in the same seat for both games.
Race tracks host far fewer events than baseball stadiums, so they're annual planned budget can't include any freebies. With racing, the fans payed a lot less for those supporting race tickets than for the Cup event. The track isn't going to let those X-only ticket holders get the Cup race for free. There's also the potential problem of different people holding tickets for the same seat in each race.
I've read of a couple of unplanned doubles where the Cup ticket holders are let in for free after 20% or 25% of the supporting race has been run. I recall M'ville started a Truck race on a Sunday double-header and then let the Cup holders in after 50 laps of 250. I guess the track figures that by that point, they've sold all the tickets they can reasonably expect to move, and the good will from letting the Cup crowd in the smart long-term move.