I agree that the scenario you describe might justifiably raise some eyebrows. Especially if this happens more than once. Were that the case then the competitors, fans and sponsors might be correct to cry foul. If it happened often enough then clearly they need to go elsewhere to enjoy the sport.
But based on this one instance, I would ask several questions:
1. Did the other competitors let him win?
2. Is there any evidence of cheating or benefiting unfairly that is being overlooked because of his position as owner of the series?
3. Is Stewart running equipment that is legal but capable of performance that is beyond what the other competitors can afford?
4. What was the reason for offering a prize five times the usual award?
5. Why did Stewart choose to compete in this particular event when he isn't a regular entrant?
6. Was winning an extra $20,000 worth the potential damage to Stewart's reputation and the continued success of his series?
If there is any serious suggestion of impropriety then Stewart would be well advised to stay off the track or risk losing what he has invested in the series.Then there is the the issue of being a big fish swimming in a small pond. His participation may be a draw for spectators but if he dominates every race people will quickly lose interest.
I know nothing of the series or of Stewart's motivation so that is all I can offer. Yes, there is potential for impropriety but it would also be potentially self-defeating.
It is an interesting parallel to the new arrangement with Penske owning the 500, the Speedway and the series. The biggest difference is that Penske, as an entrant, is a big fish in a big pond. All the teams use the same equipment and are on a similar footing, subject to sponsorship, commitment and talent. There is a great deal at stake here. Anything questionable will be immediately questioned.
I would also suggest that if there is any serious suggestion of impropriety in this scenario that Penske stop entering cars in Indycar racing. But that possibility strikes me as extremely unlikely. If it came to that the Indycar series would already be in dire straights.
But we ourselves can monitor the situation closely. As I pointed out earlier, Penske's cars have won 35.3% of the Indy 500's since he started racing there in 1969 without owning the race, the track or the series. (Already seems unfair doesn't it!) Don't know what the percentage is for the series but if his percentages suddenly jump up towards 100% we will know that buggery is afoot and we can all take to the streets in protest.