It does not sound like your manager is in line to be manager of the month. He or she has taken a motivational program and totally undermined its meaning and effectiveness.
Unless there was a dire emergency, whoever owned the car should have been advised to move it. Short of that, you should be given an apology plus the place for an extra day.
When companies put in motivational programs and then fail to support them, all sorts of negative outcomes occur. For example, this type of treatment is demotivational not only to the impacted employee, but to his or her co-workers as well. People hear about what happened and they figure that it is simply not worth it to put forth the effort to become employee of the month.
In terms of the bigger picture, the company's credibility is undercut as well. The company made a commitment to the winning employee that he or she would have this special place, and then the company had no problem ignoring what it promised. If the company can't even be trusted in the parking lot, employees will wonder if it can be trusted anywhere.
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